Related
I'm trying to execute while loop until the user enter Yes or No.
Why can't there be an OR in the testing function instead of AND?
While using OR, it can be either yes or no.
System.out.print("Enter yes or no: ");
String answer = input.getNext();
while (!answer.equals("yes") && !answer.equals("no")) {
System.out.println("Enter ONLY yes or no, please: ");
answer = input.getNext();
}
System.out.println("Thank you!");
Because !answer.equals("yes") && !answer.equals("no") basically says while answer is not "yes" and answer is not "no".
In order to make it an OR you can do it like:
while(!(answer.equals("yes") || answer.equals("no")))
{
}
Which means, while the following condition is not true; condition: answer is "yes" or answer is "no"
while (!answer.equals("yes") && !answer.equals("no")) means that the loop terminates when the condition becomes false. According to de Morgan laws, !(!answer.equals("yes") && !answer.equals("no")) is answer.equals("yes") || answer.equals("no"). That is, the loop terminates when the answer is either "yes" or "no" - that's exactly what you want.
If you change && to ||, the loop would terminate when the answer is both "yes" and "no" at the same time, which is impossible.
If you use the short circuit or/and (|| and &&) the system doesn't evaluate the entire expression if it doesn't have to. So if the user enters "no", the first part that checks if it is not "yes" evaluates to true. Since an or condition only needs one true to evaluate to true it doesn't evaluate anything else and just returns.
You need && because you are saying if the answer is not yes and is not no ask for input again. Here is more on short circuit boolean operators: https://users.drew.edu/bburd/JavaForDummies4/ShortCircuitEval.pdf.
I have been working with Java a couple of years, but up until recently I haven't run across this construct:
int count = isHere ? getHereCount(index) : getAwayCount(index);
This is probably a very simple question, but can someone explain it? How do I read it? I am pretty sure I know how it works.
if isHere is true, getHereCount() is called,
if isHere is false getAwayCount() is called.
Correct? What is this construct called?
Yes, it is a shorthand form of
int count;
if (isHere)
count = getHereCount(index);
else
count = getAwayCount(index);
It's called the conditional operator. Many people (erroneously) call it the ternary operator, because it's the only ternary (three-argument) operator in Java, C, C++, and probably many other languages. But theoretically there could be another ternary operator, whereas there can only be one conditional operator.
The official name is given in the Java Language Specification:
§15.25 Conditional Operator ? :
The conditional operator ? : uses the boolean value of one expression to decide which of two other expressions should be evaluated.
Note that both branches must lead to methods with return values:
It is a compile-time error for either the second or the third operand expression to be an invocation of a void method.
In fact, by the grammar of expression statements (§14.8), it is not permitted for a conditional expression to appear in any context where an invocation of a void method could appear.
So, if doSomething() and doSomethingElse() are void methods, you cannot compress this:
if (someBool)
doSomething();
else
doSomethingElse();
into this:
someBool ? doSomething() : doSomethingElse();
Simple words:
booleanCondition ? executeThisPartIfBooleanConditionIsTrue : executeThisPartIfBooleanConditionIsFalse
Others have answered this to reasonable extent, but often with the name "ternary operator".
Being the pedant that I am, I'd like to make it clear that the name of the operator is the conditional operator or "conditional operator ?:". It's a ternary operator (in that it has three operands) and it happens to be the only ternary operator in Java at the moment.
However, the spec is pretty clear that its name is the conditional operator or "conditional operator ?:" to be absolutely unambiguous. I think it's clearer to call it by that name, as it indicates the behaviour of the operator to some extent (evaluating a condition) rather than just how many operands it has.
According to the Sun Java Specification, it's called the Conditional Operator. See section 15.25. You're right as to what it does.
The conditional operator ? : uses the boolean value of one expression to decide which of two other expressions should be evaluated.
The conditional operator is syntactically right-associative (it groups right-to-left), so that a?b:c?d:e?f:g means the same as a?b:(c?d:(e?f:g)).
ConditionalExpression:
ConditionalOrExpression
ConditionalOrExpression ? Expression : ConditionalExpression
The conditional operator has three operand expressions; ? appears between the first and second expressions, and : appears between the second and third expressions.
The first expression must be of type boolean or Boolean, or a compile-time error occurs.
condition ? truth : false;
If the condition is true then evaluate the first expression. If the condition is false, evaluate the second expression.
It is called the Conditional Operator and it is a type of Ternary Operation.
int count = isHere ? getHereCount(index) : getAwayCount(index);
means :
if (isHere) {
count = getHereCount(index);
} else {
count = getAwayCount(index);
}
Not exactly correct, to be precise:
if isHere is true, the result of getHereCount() is returned
otheriwse the result of getAwayCount() is returned
That "returned" is very important. It means the methods must return a value and that value must be assigned somewhere.
Also, it's not exactly syntactically equivalent to the if-else version. For example:
String str1,str2,str3,str4;
boolean check;
//...
return str1 + (check ? str2 : str3) + str4;
If coded with if-else will always result in more bytecode.
Ternary, conditional; tomato, tomatoh. What it's really valuable for is variable initialization. If (like me) you're fond of initializing variables where they are defined, the conditional ternary operator (for it is both) permits you to do that in cases where there is conditionality about its value. Particularly notable in final fields, but useful elsewhere, too.
e.g.:
public class Foo {
final double value;
public Foo(boolean positive, double value) {
this.value = positive ? value : -value;
}
}
Without that operator - by whatever name - you would have to make the field non-final or write a function simply to initialize it. Actually, that's not right - it can still be initialized using if/else, at least in Java. But I find this cleaner.
You might be interested in a proposal for some new operators that are similar to the conditional operator. The null-safe operators will enable code like this:
String s = mayBeNull?.toString() ?: "null";
It would be especially convenient where auto-unboxing takes place.
Integer ival = ...; // may be null
int i = ival ?: -1; // no NPE from unboxing
It has been selected for further consideration under JDK 7's "Project Coin."
This construct is called Ternary Operator in Computer Science and Programing techniques. And Wikipedia suggest the following explanation:
In computer science, a ternary operator (sometimes incorrectly called a tertiary operator) is an operator that takes three arguments. The arguments and result can be of different types. Many programming languages that use C-like syntax feature a ternary operator, ?: , which defines a conditional expression.
Not only in Java, this syntax is available within PHP, Objective-C too.
In the following link it gives the following explanation, which is quiet good to understand it:
A ternary operator is some operation operating on 3 inputs. It's a shortcut for an if-else statement, and is also known as a conditional operator.
In Perl/PHP it works as: boolean_condition ? true_value : false_value
In C/C++ it works as: logical expression ? action for true : action for false
This might be readable for some logical conditions which are not too complex otherwise it is better to use If-Else block with intended combination of conditional logic.
We can simplify the If-Else blocks with this Ternary operator for one code statement line.For Example:
if ( car.isStarted() ) {
car.goForward();
} else {
car.startTheEngine();
}
Might be equal to the following:
( car.isStarted() ) ? car.goForward() : car.startTheEngine();
So if we refer to your statement:
int count = isHere ? getHereCount(index) : getAwayCount(index);
It is actually the 100% equivalent of the following If-Else block:
int count;
if (isHere) {
count = getHereCount(index);
} else {
count = getAwayCount(index);
}
That's it!
Hope this was helpful to somebody!
Cheers!
Correct. It's called the ternary operator. Some also call it the conditional operator.
Its Ternary Operator(?:)
The ternary operator is an operator that takes three arguments. The first
argument is a comparison argument, the second is the result upon a true
comparison, and the third is the result upon a false comparison.
Actually it can take more than 3 arguments. For instance if we want to check wether a number is positive, negative or zero we can do this:
String m= num > 0 ? "is a POSITIVE NUMBER.": num < 0 ?"is a NEGATIVE NUMBER." :"IT's ZERO.";
which is better than using if, else if, else.
?: is a Ternary Java Operator.
Its syntax is:
condition ? expression1 : expression2;
Here, the condition is evaluated and
condition returns true, the expression1 will execute.
condition returns false, the expression2 will execute.
public class Sonycode {
public static void main(String[] args) {
double marks = 90;
String result = (marks > 40) ? "passed in exam" : "failed in exam";
System.out.println("Your result is : " + result);
}
}
Output :-
Your result is : passed in exam
It's the conditional operator, and it's more than just a concise way of writing if statements.
Since it is an expression that returns a value it can be used as part of other expressions.
Yes, you are correct. ?: is typically called the "ternary conditional operator", often referred to as simply "ternary operator". It is a shorthand version of the standard if/else conditional.
Ternary Conditional Operator
I happen to really like this operator, but the reader should be taken into consideration.
You always have to balance code compactness with the time spent reading it, and in that it has some pretty severe flaws.
First of all, there is the Original Asker's case. He just spent an hour posting about it and reading the responses. How longer would it have taken the author to write every ?: as an if/then throughout the course of his entire life. Not an hour to be sure.
Secondly, in C-like languages, you get in the habit of simply knowing that conditionals are the first thing in the line. I noticed this when I was using Ruby and came across lines like:
callMethodWhatever(Long + Expression + with + syntax) if conditional
If I was a long time Ruby user I probably wouldn't have had a problem with this line, but coming from C, when you see "callMethodWhatever" as the first thing in the line, you expect it to be executed. The ?: is less cryptic, but still unusual enough as to throw a reader off.
The advantage, however, is a really cool feeling in your tummy when you can write a 3-line if statement in the space of 1 of the lines. Can't deny that :) But honestly, not necessarily more readable by 90% of the people out there simply because of its' rarity.
When it is truly an assignment based on a Boolean and values I don't have a problem with it, but it can easily be abused.
Conditional expressions are in a completely different style, with no explicit if in the statement.
The syntax is:
boolean-expression ? expression1 : expression2;
The result of this conditional expression is
expression1 if boolean-expression is true;
otherwise the result is expression2.
Suppose you want to assign the larger number of variable num1 and num2 to max. You can simply write a statement using the conditional expression:
max = (num1 > num2) ? num1 : num2;
Note: The symbols ? and : appear together in a conditional expression. They form a conditional operator and also called a ternary operator because it uses three operands. It is the only ternary operator in Java.
cited from: Intro to Java Programming 10th edition by Y. Daniel Liang page 126 - 127
String c = JOptionPane.showInputDialog(null,"Pick a conversion to do:\nGrade");
if (c.equals("grade")||("Grade"))
I want it to start if the user enters grade or Grade not just grade, but when I use this I get a error. How would I fix that
The actual problem: You're using || improperly. It expects, on both sides, something of type boolean to be present.
From the JLS:
Each operand of the conditional-or operator must be of type boolean or Boolean, or a compile-time error occurs.
Since "Grade" isn't a boolean (but c.equals("grade") is), this becomes a compile-time error.
While you could fix it by correcting the boolean on the right hand side...
if(c.equals("grade") || c.equals("Grade"))
...why not do it in a more straightforward way?
if(c.equalsIgnoreCase("grade"))
This particular equals on String was likely engineered for this use case (or other use cases, as "GrAde" would fail your check too). It will perform a length check on your strings first, then a case insensitive comparison on each element.
You cannot put an || statement in the middle of a method call like that.
if (c.equals("grade")||c.equals("Grade")){
//do stuff
}
There are other ways to accomplish your task, IE using other methods, but your problem is coming form a misuse of the || operator.
You can try:
if (c.toLowerCase().equals("grade")) { }
This question already has answers here:
How do I compare strings in Java?
(23 answers)
Closed 9 years ago.
I was working on my game I am making, when I came across an error. My if/else if statement skips right to the else if statement, even if it shouldn't.
String neededCredits = "200";
if(Peeamnt.getText() == neededCredits) {
System.out.println("You can afford this");
JOptionPane.showMessageDialog(BuyPoopButton,
"You have unlocked the Poop Button for 200 Pee Credits!",
"Toilet Master",
JOptionPane.WARNING_MESSAGE);
}
else if((!(Peeamnt.getText() == neededCredits))) {
System.out.println("You cannot afford this");
JOptionPane.showMessageDialog(BuyPoopButton,
"You do not have enough Credits to buy this!\n"
+ "To buy it, you need 200 Pee Credits!",
"Toilet Master",
JOptionPane.ERROR_MESSAGE);
}
Even if the text of Peeamnt is an even 200, the code will jump to the else if statement, telling me that I don't have 200 Pee Credits. (The game I am making included a lot of toilet humor.) Anyway, if anyone sees the error I have in this code, please let me know. Let me know if you need more code.
With a Java String object, the == operator doesn't compare the string value.
Try changing the first if comparison to:
if(Peeamnt.getText().equals(neededCredits)) {
You will need to do something similar for the else if as well.
Strings are objects. Objects have a reference. Two String objects containing the same sequence of characters may not be the same object, thus having different references. The == operator (generally) checks for reference equality.
To compare the character sequence of two String objects for equality, you have the equals method. So use Peeamnt.getText().equals(neededCredits) instead.
String is an Object. Comparing Object, you have to use equals to judge whether the Object content is same. Using == is to compare Object reference
Use equals method to compare String object, because == operator means you compare object base on memory address. Always remember to never use == to compare objects in Java.
Try to use equals method if the getText() returns a string don 't use == sign. I suppose that getText() returns aString` object.
FYI: Double equal sign is used to see if two Objects are the same
and TO check if the objects has the same value equals() method should be used. Note: The objects that You compare with equals() should override it otherwise the results are corrupted, and the last thing when You overridesequals() method remember to override hashCode() too.
Use method equals to compare String object, because == operator means you compare object base on memory address.
* never use == to compare object.
String neededCredits = "200";
if(neededCredits.equals(Peeamnt.getText()) {//compare following you never see, because nullPointerException "neededCredits" always has value :-)
System.out.println("You can afford this");
JOptionPane.showMessageDialog(BuyPoopButton,
"You have unlocked the Poop Button for 200 Pee Credits!",
"Toilet Master",
JOptionPane.WARNING_MESSAGE);
}
else if((!(neededCredits.equals(Peeamnt.getText()))) {
System.out.println("You cannot afford this");
JOptionPane.showMessageDialog(BuyPoopButton,
"You do not have enough Credits to buy this!\n"
+ "To buy it, you need 200 Pee Credits!",
"Toilet Master",
JOptionPane.ERROR_MESSAGE);
}
I have been working with Java a couple of years, but up until recently I haven't run across this construct:
int count = isHere ? getHereCount(index) : getAwayCount(index);
This is probably a very simple question, but can someone explain it? How do I read it? I am pretty sure I know how it works.
if isHere is true, getHereCount() is called,
if isHere is false getAwayCount() is called.
Correct? What is this construct called?
Yes, it is a shorthand form of
int count;
if (isHere)
count = getHereCount(index);
else
count = getAwayCount(index);
It's called the conditional operator. Many people (erroneously) call it the ternary operator, because it's the only ternary (three-argument) operator in Java, C, C++, and probably many other languages. But theoretically there could be another ternary operator, whereas there can only be one conditional operator.
The official name is given in the Java Language Specification:
§15.25 Conditional Operator ? :
The conditional operator ? : uses the boolean value of one expression to decide which of two other expressions should be evaluated.
Note that both branches must lead to methods with return values:
It is a compile-time error for either the second or the third operand expression to be an invocation of a void method.
In fact, by the grammar of expression statements (§14.8), it is not permitted for a conditional expression to appear in any context where an invocation of a void method could appear.
So, if doSomething() and doSomethingElse() are void methods, you cannot compress this:
if (someBool)
doSomething();
else
doSomethingElse();
into this:
someBool ? doSomething() : doSomethingElse();
Simple words:
booleanCondition ? executeThisPartIfBooleanConditionIsTrue : executeThisPartIfBooleanConditionIsFalse
Others have answered this to reasonable extent, but often with the name "ternary operator".
Being the pedant that I am, I'd like to make it clear that the name of the operator is the conditional operator or "conditional operator ?:". It's a ternary operator (in that it has three operands) and it happens to be the only ternary operator in Java at the moment.
However, the spec is pretty clear that its name is the conditional operator or "conditional operator ?:" to be absolutely unambiguous. I think it's clearer to call it by that name, as it indicates the behaviour of the operator to some extent (evaluating a condition) rather than just how many operands it has.
According to the Sun Java Specification, it's called the Conditional Operator. See section 15.25. You're right as to what it does.
The conditional operator ? : uses the boolean value of one expression to decide which of two other expressions should be evaluated.
The conditional operator is syntactically right-associative (it groups right-to-left), so that a?b:c?d:e?f:g means the same as a?b:(c?d:(e?f:g)).
ConditionalExpression:
ConditionalOrExpression
ConditionalOrExpression ? Expression : ConditionalExpression
The conditional operator has three operand expressions; ? appears between the first and second expressions, and : appears between the second and third expressions.
The first expression must be of type boolean or Boolean, or a compile-time error occurs.
condition ? truth : false;
If the condition is true then evaluate the first expression. If the condition is false, evaluate the second expression.
It is called the Conditional Operator and it is a type of Ternary Operation.
int count = isHere ? getHereCount(index) : getAwayCount(index);
means :
if (isHere) {
count = getHereCount(index);
} else {
count = getAwayCount(index);
}
Not exactly correct, to be precise:
if isHere is true, the result of getHereCount() is returned
otheriwse the result of getAwayCount() is returned
That "returned" is very important. It means the methods must return a value and that value must be assigned somewhere.
Also, it's not exactly syntactically equivalent to the if-else version. For example:
String str1,str2,str3,str4;
boolean check;
//...
return str1 + (check ? str2 : str3) + str4;
If coded with if-else will always result in more bytecode.
Ternary, conditional; tomato, tomatoh. What it's really valuable for is variable initialization. If (like me) you're fond of initializing variables where they are defined, the conditional ternary operator (for it is both) permits you to do that in cases where there is conditionality about its value. Particularly notable in final fields, but useful elsewhere, too.
e.g.:
public class Foo {
final double value;
public Foo(boolean positive, double value) {
this.value = positive ? value : -value;
}
}
Without that operator - by whatever name - you would have to make the field non-final or write a function simply to initialize it. Actually, that's not right - it can still be initialized using if/else, at least in Java. But I find this cleaner.
You might be interested in a proposal for some new operators that are similar to the conditional operator. The null-safe operators will enable code like this:
String s = mayBeNull?.toString() ?: "null";
It would be especially convenient where auto-unboxing takes place.
Integer ival = ...; // may be null
int i = ival ?: -1; // no NPE from unboxing
It has been selected for further consideration under JDK 7's "Project Coin."
This construct is called Ternary Operator in Computer Science and Programing techniques. And Wikipedia suggest the following explanation:
In computer science, a ternary operator (sometimes incorrectly called a tertiary operator) is an operator that takes three arguments. The arguments and result can be of different types. Many programming languages that use C-like syntax feature a ternary operator, ?: , which defines a conditional expression.
Not only in Java, this syntax is available within PHP, Objective-C too.
In the following link it gives the following explanation, which is quiet good to understand it:
A ternary operator is some operation operating on 3 inputs. It's a shortcut for an if-else statement, and is also known as a conditional operator.
In Perl/PHP it works as: boolean_condition ? true_value : false_value
In C/C++ it works as: logical expression ? action for true : action for false
This might be readable for some logical conditions which are not too complex otherwise it is better to use If-Else block with intended combination of conditional logic.
We can simplify the If-Else blocks with this Ternary operator for one code statement line.For Example:
if ( car.isStarted() ) {
car.goForward();
} else {
car.startTheEngine();
}
Might be equal to the following:
( car.isStarted() ) ? car.goForward() : car.startTheEngine();
So if we refer to your statement:
int count = isHere ? getHereCount(index) : getAwayCount(index);
It is actually the 100% equivalent of the following If-Else block:
int count;
if (isHere) {
count = getHereCount(index);
} else {
count = getAwayCount(index);
}
That's it!
Hope this was helpful to somebody!
Cheers!
Correct. It's called the ternary operator. Some also call it the conditional operator.
Its Ternary Operator(?:)
The ternary operator is an operator that takes three arguments. The first
argument is a comparison argument, the second is the result upon a true
comparison, and the third is the result upon a false comparison.
Actually it can take more than 3 arguments. For instance if we want to check wether a number is positive, negative or zero we can do this:
String m= num > 0 ? "is a POSITIVE NUMBER.": num < 0 ?"is a NEGATIVE NUMBER." :"IT's ZERO.";
which is better than using if, else if, else.
?: is a Ternary Java Operator.
Its syntax is:
condition ? expression1 : expression2;
Here, the condition is evaluated and
condition returns true, the expression1 will execute.
condition returns false, the expression2 will execute.
public class Sonycode {
public static void main(String[] args) {
double marks = 90;
String result = (marks > 40) ? "passed in exam" : "failed in exam";
System.out.println("Your result is : " + result);
}
}
Output :-
Your result is : passed in exam
It's the conditional operator, and it's more than just a concise way of writing if statements.
Since it is an expression that returns a value it can be used as part of other expressions.
Yes, you are correct. ?: is typically called the "ternary conditional operator", often referred to as simply "ternary operator". It is a shorthand version of the standard if/else conditional.
Ternary Conditional Operator
I happen to really like this operator, but the reader should be taken into consideration.
You always have to balance code compactness with the time spent reading it, and in that it has some pretty severe flaws.
First of all, there is the Original Asker's case. He just spent an hour posting about it and reading the responses. How longer would it have taken the author to write every ?: as an if/then throughout the course of his entire life. Not an hour to be sure.
Secondly, in C-like languages, you get in the habit of simply knowing that conditionals are the first thing in the line. I noticed this when I was using Ruby and came across lines like:
callMethodWhatever(Long + Expression + with + syntax) if conditional
If I was a long time Ruby user I probably wouldn't have had a problem with this line, but coming from C, when you see "callMethodWhatever" as the first thing in the line, you expect it to be executed. The ?: is less cryptic, but still unusual enough as to throw a reader off.
The advantage, however, is a really cool feeling in your tummy when you can write a 3-line if statement in the space of 1 of the lines. Can't deny that :) But honestly, not necessarily more readable by 90% of the people out there simply because of its' rarity.
When it is truly an assignment based on a Boolean and values I don't have a problem with it, but it can easily be abused.
Conditional expressions are in a completely different style, with no explicit if in the statement.
The syntax is:
boolean-expression ? expression1 : expression2;
The result of this conditional expression is
expression1 if boolean-expression is true;
otherwise the result is expression2.
Suppose you want to assign the larger number of variable num1 and num2 to max. You can simply write a statement using the conditional expression:
max = (num1 > num2) ? num1 : num2;
Note: The symbols ? and : appear together in a conditional expression. They form a conditional operator and also called a ternary operator because it uses three operands. It is the only ternary operator in Java.
cited from: Intro to Java Programming 10th edition by Y. Daniel Liang page 126 - 127