comparing string literals and string variables... 2

[mackey333]

I know this is a stupid question, but why can I not compare a string literal with a string variable? For instance, compile and run this program. If you enter "blah" as prompted, it will return a false value.:

import javax.swing.*;
import java.io.*;

class test
{
	public static void main(String args[])
	throws java.io.IOException
	{
		String testIt;
		
		testIt = JOptionPane.showInputDialog("Enter 'blah':");
		
		if (testIt == "blah")
		{
			System.out.println("The values are equal.");
		}
		else
		{
			System.out.println("testIt = ***" + testIt + "***");
			System.out.println("WTF???");
		}
		
		InputStreamReader isr = new InputStreamReader(System.in);
		BufferedReader br = new BufferedReader(isr);
		
		System.out.print("\n\n\nType 'blah' then hit enter: ");
		
		testIt = br.readLine();
		
		System.out.print("\n\n");
		
		if (testIt == "blah")
		{
			System.out.print("The values are equal.");
		}
		else
		{
			System.out.println("testIt = ***" + testIt + "***");
			System.out.println("WTF???");
		}
		
		System.out.print("\n\n");
		System.exit(0);
	}
}

-Greg :-Q

 

[jfryer]

When you compare two strings you have to decide if you are comparing the string object, or the string literal.

Take this code snippet

String a = "hello";
String b = "hello";

now you can do 2 things with it

a == b
a.equals(b)

a and b are both in effect pointers to where the string object associated with a and b are stored in memory. This is why the

new

operator is required.
Saying

JButton button;

creates a pointer to a memory space where the JButton object will live, it does not create the JButton object.
Calling

button = new JButton();

creates the object in the memory space and associates the pointer to it, so when the virtual machine looks at the

button

variable it knows where to find the JButton object.

(I know Java does not make the use of pointers available to the programmer, but internally it does use them)

So

a == b

will return false, since what you are telling the virtual machine to do is to compare the address of object a to the address of object b, NOT the contains of a and b.
Whereas

a.equals(b)

tells the virtual machine to compare the contents of a to the contents of b.

so to test for "blah" you need to type

testIt.equals("blah");

or

testIt.equalsIgnoreCase("blah");

I hope that clears it up a bit (let me know if it doesn't). ----------------------------------------
There are no onions, only magic
----------------------------------------

 

[MarsChelios]

You are a little off, jfryer. Strings are highly weird when checking equality, and it depends how the String was made and how you check for equality.

Example 1 "String literal":
String someString1 = "MarsChelios";
boolean equals1 = (someString1 == "MarsChelios"
boolean equals2 = someString1.equals ("MarsChelios"

Result: equals1 is true, equals2 is true
Reason: When String literals are defined, they are stored in a String Table somewhere. Any other String literal will equal true when compared to it either way because for all intents and purposes, they are the same object.

Example 2 "Instantiated Strings":
String someString1 = new String ("MarsChelios"
boolean equals1 = (someString1 == "MarsChelios"
boolean equals2 = someString1.equals ("MarsChelios"

Result: equals1 is false, equals2 is true
Reason: When Objects are instantiated, they are given a unique id, their refernce id. This is how the equality operator == check equality of objects. The instantiated String has a different id than the String literal, and so fails the equality operator check.
However, the equals () method of String checks the chracters in a String for equality, so the check passes here.

In your example, you use 2 String literals which will compare to true. It's when you or Java actually instantiate a String object with

new

that the reference changes and comparison with a literal String will fail.

Hope this helps,
MarsChelios

 

[mackey333]

thanks -Greg :-Q

 

[Tarwn]

Nice --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- ---
For my next trick I will pull a hat out of a rabbit (if you think thats bad you should see how the pigeon feels...)