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Closed 10 years ago.
Possible Duplicate:
How do I compare strings in Java?
So, I have a question regarding the testing operators and strings. I'm trying to write a program that will take in user input of either "boy" or "girl". When the user inputs "boy", the output should be "You are a boy." When the user inputs "girl", the output should be "You are a girl."
However, when I compile and run the program, no matter what I input, the output is always "You are a girl."
Is this because the strings have no actual value like integers and therefore, testing operators cannot be used to compare them?
Also, is there anything like the assert function from python in java?
import java.util.Scanner;
class apples{
public static void main(String args[]){
System.out.println("Are you a boy or a girl?");
Scanner lalala = new Scanner(System.in);
String text = lalala.nextLine();
if (text == "boy"){
System.out.println("You are a boy.");
}
else{
System.out.println("You are a girl.");
}
}
}
Thanks a lot~
Use
text.equals("boy")
instead of
if (text == "boy")
The reason is, In Java, == always just compares two references (for non-primitives, that is) - i.e. it tests whether the two operands refer to the same object.
However, the equals method can be overridden - so two distinct objects can still be equal.
But the better option is to use equalsIgnoreCase, the reason is that user may enter boy or BOY or Boy or any other combination. equalsIgnoreCase method just ignores the case and compares two strings.
text.equalsIgnoreCase("boy")
Side note. Input other than Boy will get you to print Girl. Put one more extra if condition. Also before going for comparision, being on safer side trim the string for blank spaces.
Use
text.equals("boy")
or
text.equalIgnoreCase("boy")
if its not case sensitive.
instead of
if (text == "boy")
Your coade will be something like this
import java.util.Scanner;
class apples{
public static void main(String args[]){
System.out.println("Are you a boy or a girl?");
Scanner lalala = new Scanner(System.in);
String text = lalala.nextLine();
text = text.trim();
if (text.equalIgnoreCase("Boy")){
System.out.println("You are a boy.");
}
else if(text.equalIgnoreCase("Girl")){
System.out.println("You are a girl.");
} else {
System.out.println("Invalid Gender");
}
}
}
use text.equalsIgnoreCase("boy")
if you think about case also it will check content of string is same.
== is used to check for object equality.
text.equals("boy") is ok if you not consider about case.
Please use equals method for string like : if (text.equals("boy")){
== is not for string contents equality check in java
== tests if object identity is the same. When you have two string objects containing the same value this object identity won't be equal. Use the equals function to test for logical equivalence.
Your if else is wrong. Use only if here. Otherwise,
If I entered "some text", the output will be "You are a girl"
Like this
if (text.equalsIgnoreCase("boy")) {
System.out.println("You are a boy.");
}
else if (text.equalsIgnoreCase("girl")) {
System.out.println("You are a girl.");
} else {
System.out.println("You are neither a boy nor a girl");
}
It would be best to use the .equals method instead of == boy
For your other question regarding assert:
Java has an assert function. This is the sample code.
assert(x <= 50): "Not Accepted";
This code checks if integer x is greater than 50. You will see the output at the console.
Related
This question already has answers here:
What is the difference between == and equals() in Java?
(26 answers)
Closed 10 months ago.
I am within my first year of CS and near the end of my first Java themed course so I'm not quite sure how to find the answer to my question myself.
While writing some code for a project I created my input scanner as:
Scanner scanner = new Scanner(System.in);
I am taking user inputs as strings via a variable assignment:
String userInput = scanner.nextLine();
the user should only be entering strings of char "1" - "6" and "q" (to quit app)
What I'm using that works currently is as follows:
userInput = scanner.nextLine();
while (!appQuit) { //So long as user doesn't quit application
if (userInput.equals("q")) {
appQuit = true;
}
else if (userInput.equals("1")) { //Menu selection for intake a new dog
intakeNewDog(scanner);
displayMenu();
userInput = scanner.nextLine();
}
//removed "2" - "6" for brevity
else {
System.out.println("Not a valid input");
displayMenu();
userInput = scanner.nextLine();
}
}
The only way I found to check equality was the userInput.equals() function.
When I originally wrote it I tried using:
if (userInput == "1") { code }
but it would never successfully compare values as I thought it would.
Any insight into why one method works over the other? Or where I should be looking for these answers?
-Jonesy
The == equal operator compares the object references where the equals function compares the value.
For primitive types and enums the == equal operator compares the value.
An exception happens for comparing strings in a switch case statement since it internal uses the equals method.
As a rule of thumb, always use equals comparison for String. There maybe is, but i have not seen a case where reference comparison was important.
https://docs.oracle.com/javase/8/docs/technotes/guides/language/strings-switch.html
Also interesting:
What makes reference comparison (==) work for some strings in Java?
In simple words, == checks if both objects point to the same memory location whereas . equals() evaluates to the comparison of values in the objects. You can override the equals method to do more specific things, but that's the just of it.
This is literally the first thing that appears if you search java == vs equals in google.
While you might be trying to compare two strings, the operator == does not behave in java as it does in other languages.
This question already has answers here:
How do I compare strings in Java?
(23 answers)
Closed 8 years ago.
I wrote a simple program which prompts the user to enter a sentence, word, or number, and then prints input is art. I put the program on an infinite loop so that the prompt would repeat. However, now I'm trying to add a way to quit by saying if (input == "quit") break; however it does not seem to be working. It just continues the infinite loop. Here is the full code:
import java.util.Scanner;
public class Art
{
public static void main(String [] args)
{
Scanner input = new Scanner(System.in);
String a;
for ( ; ; )
{
System.out.println("Please enter something: ");
a = input.nextLine();
if (a == "quit") break;
System.out.print("\n");
System.out.println(a + " is art.");
System.out.print("\n");
}
}
}
Any help would be lovely, thanks!
Use input.equals("quit").
The == is used to check whether two objects are the exact same instance - the same thing in memory -which the typed word and the constant string "quit" are not. Two instances of "quit" were created: one typed by the user, and one constant in the program.
The equals() method is used to compare whether two objects are equal in whatever way equality is defined for them. For strings, that would mean having the same text.
The difference between == and equals() is really fundamental in Java, so you should go over it. Here's a good SO post on the topic. Once you start creating your own classes, you will probably be implementing equals() methods for them. For that reason, make sure to go over the equals() / hashCode() contract as well.
looks like the a == "quit" is the problem. Compare strings using equals()
if("quit".equals(a)) {
}
This question already has answers here:
How do I compare strings in Java?
(23 answers)
Closed 9 years ago.
So, for some reason I'm having problems just USING a string input.
I don't know why. Maybe it's some incredibly stupid thing everyone knows, but I don't.
Here's the non-functioning code:
import javax.swing.*;
public class Thing {
public static void main(String[] args) {
String input;
JOptionPane.showMessageDialog(null,"Welcome to the test...");
input = JOptionPane.showInputDialog("Do you wish to take the tutorial?" + "\n" +
"If affirmative, enter 'Yes'");
String i = input;
if(i == "Yes") {
tutorial();
} else if(input=="'Yes'") {
JOptionPane.showMessageDialog(null,"Don't actually put apostraphes around you're answer.");
tutorial();
} else {
JOptionPane.showMessageDialog(null,"Remember, you can pull up the tutorial at any time with 'T'");
}
}
Yes, I actually do have a tutorial method somewhere else, and it works fine.
The main problem is that if I enter 'Yes' or Yes, it still goes to the final else.
I only put in the
String i = input;
and changed it from
if(input == "Yes") {
because it didn't work then, either.
So what am I doing wrong?
Don't use the == operator to compare Strings, use equals() instead, as thoroughly explained here, here, here, here or any of the numerous duplicates.
if ("Yes".equals(input))
Or even
if ("yes".equalsIgnoreCase(input))
Notice that the operation is invoked on the "yes" literal to avoid a possible NullPointerException in the case input was null and the operation was invoked on it (Yoda condition).
From the Java Language Specification, Chapter 15 - Expressions, section 21 - Equality Operators:
15.21.3. Reference Equality Operators == and !=
While == may be used to compare references of type String, such an equality test determines whether or not the two operands refer to the same String object. The result is false if the operands are distinct String objects, even if they contain the same sequence of characters (ยง3.10.5). The contents of two strings s and t can be tested for equality by the method invocation s.equals(t).
As mentioned, the problem is that you are comparing this String using the == comparator, not the .equals() method.
If you are running on Java 7, my advice, for a cleaner solution, would be also to wrap this in a switch statement:
JOptionPane.showMessageDialog(null,"Welcome to the test...");
String input = JOptionPane.showInputDialog("Do you wish to take the tutorial?" + "\n" +
"If affirmative, enter 'Yes'");
switch (input) {
case "Yes":
tutorial();
break;
case "'Yes'":
JOptionPane.showMessageDialog(null,"Don't actually put apostraphes around you're answer.");
tutorial();
break;
default:
JOptionPane.showMessageDialog(null,"Remember, you can pull up the tutorial at any time with 'T'");
}
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Java String.equals versus ==
I have been having difficulties using nextLine() to get a string, and then use it as a test condition (either in an if statement or a while loop). Looking at the println(), it seems as if the String is correctly assigned to the variable 'repeat' but then the test condition fails for some reason. Banging my head on the wall, bleeding from my forehead. Please help.
import java.util.Scanner;
public class potpie {
public static void main(String args[]){
Scanner input = new Scanner(System.in);
String repeat = "yes";
System.out.println("Type in yes");
repeat = input.nextLine();
System.out.println("If repeat is now yes, print yes: " +repeat);
if(repeat == "yes"){
System.out.println("It worked");
} else
System.out.println("it failed");
}
}
You should use equals. == provide you reference equality and equals provide you value equality.
if("yes".equals(repeat)){
instead of
if(repeat == "yes"){
I would advice you to get eclipse/net beans and start debugging or a simple search would have resulted in the answer
Java Debugging with Eclipse - Tutorial
if(repeat == "yes"){
should be
if(repeat.equals("yes"){
(or)
if("yes".equals(repeat){
Every day we see this question lot of times, simple search could have provided you sufficient information.
== equals for primitive comparison (reference equality). equals() is for String (or) Object comparison (object content equality).
Sometimes == should be used for objects, but what it is actually comparing is whether a and b are literally the same object (have the same address in memory). As the others have said, you are comparing content in this situation, so you use .equals()
This question already has answers here:
How do I compare strings in Java?
(23 answers)
Closed 5 years ago.
public void play () {
int anInteger;
//guess return code
int code;
while (true) {
String input=null;
input = JOptionPane.showInputDialog("Please enter an integer");
if (input == "-1") {
//JOptionPane.showMessageDialog(null, input);
System.exit(0);
break;
} else {
if (input==null) {
System.exit(0);
} else if (input.isEmpty()) {
continue;
} else {
anInteger = Integer.parseInt(input);
code = this.oneGuess (anInteger);
//JOptionPane.showMessageDialog(null, anInteger);
}
}
}
}
I want, if the user enter -1, show the program will not prompt the message box any more. Above is the code I have come up with, so far. Why it doesn't work?
String comparisons does NOT work with "==" operator, use "String.equals(Object)" function
input.equals("-1");
Better way would be
"-1".equals(input);
as it also takes care of null input
You are comparing strings, which are objects, with the == operator, which checks whether two object references refer to the same object instance. Instead you should use the equals method for comparing them.
There is a difference between comparing with == and equals. The first compares pointers, the latter contents. That is probably your issue.
You compare Strings with ==, which creates a problem. You can have many different String-Objects which all show "-1". The == tests, if you have exactly the same object on the left and right side. You want to know, if the objects on the left and right sie have an equal content.
Better try
input.equalsIgnoreCase("-1");
EDIT: To answer the comment: input.equalsIgnoreCase("-1") is the same as input.equals("-1") in the case of "-1" as there are no uppercase/lowercase letters in "-1". However, I prefer equalsIgnoreCase in the case of Strings, because it is defined on String, rather than on Object. Still, as the equals-definition is overridden for the String class, it works too in this example and "ignoreCase" is not needed.