This question already has answers here:
How do I compare strings in Java?
(23 answers)
Closed 4 years ago.
I am writing a program that should close the console if the user input the String "end'.
The program always performs the else loop even if the user inputs "end". I'm wondering why the program is not getting into the if part of the loop and shutting down.
Scanner scan = new Scanner(System.in);
while(true)
{
String num = scan.nextLine();
if(num == "end")
{
System.exit(0);
}
else
{
System.out.println("hi");
}
}
You're using == instead of "end".equals(num)
Don't use == for equality of string as it compares the objects not the string itself.
Use num.equals("end") or num.equalsIgnoreCase("end") if you want to be able to type end or END
I would not use "end".equals(num), although considered better from a performance perspective in most cases, it does not clearly state the business requirement and it is more important to make it more readable.
But be aware of num being null, if that is possible, num.quals("end") could throw an exception and you should write if (num!=null && num.equals("end")) { ... }
Note that "end".equals(num) does not need the null check, but I still believe this is not very readable, so I would go with if (num!=null && num.equals("end")) { ... }
For testing equality between strings, you should use equals() instead.
if(a.equals(b)) and so on.
This should help you out: http://leepoint.net/notes-java/data/expressions/22compareobjects.html
In Java, you test equality of strings with:
string1.equals(string2);
So in this case, it would be:
num.equals("end");
Or to avoid an exception of type NullPointerException:
"end".equals(num);
num refers to the object, so num == "end" should never be. You want num.equals("end")
Please do not use comparison (==) operator when comparing objects in Java. Use equals(Object) instead.
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 7 years ago.
First off, I know this might have been answered SOMEWHERE but I can't seem to search for the correct terms to get an answer. Also, I'm pretty new to coding, and obvious here, so this probably won't be the best written 'question'.
Quick backstory: I'm coding a sorting game in BlueJ(I know... shitty, but it's what we are learning in school), and for a method I'm creating for any yes/no questions I need isn't working properly. At first, I was having an issue with it allowing to to have the user input save as a String, now I'm having an issue with that String used in the if-else statement parameters. This is what I have right now:
public void userAnswer(int method) //used for yes/no questions
{
System.out.println("Please type 'y' for yes and 'n' for no.");
String answer = keyboard.next();
answer.toLowerCase();
System.out.println(answer + "worked");
if(answer == "y")
{
System.out.println("worked2");
if(method == 0)
completeOrNot();
if(method == 1)
usersMove(theArray);
}
else if(answer == "n")
{
System.out.println("worked3");
System.out.print("\n");
}
}
I'm completely stuck as to why it's not moving into the if-else statement. I test to see if it would print the String, and it will, but it won't convert it to lower case. I just don't know what to do. Any and all help would be appreciated!
When comparing Strings in Java, use the equals() method. Otherwise, you compare their memory locations if you use ==.
"hi".equals("hello") returns False
"hello".equals("hello") returns True
Don't use == to compare strings. use equals:
answer.equals("y")
Strings in Java are objects - you need to evaluate with the equals method for equality, not the == operator for reference identity:
if ("y".equals(answer)) {
// code
} else if ("n".equals(answer)) {
// code
}
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.