Strings

[Barkat Mardhani]

 
class Test {
public static void main(String[] abs) {
String s1 = "world";
String s2 = "hello" + s1;
String s3 = "hello" + s1;
String s4 = "hello" + "world";
String s5 = "hello" + "world";
System.out.println(s2 == s3); // line 1: false
System.out.println(s4== s5); // true
}
}
 

Why line 1 prints false?

[Jeff Bosch]

You're creating two different objects in the String constant pool. The equality test (==) only returns true when two object references are pointing to the same object. If you want to compare value, then use one of the String's equals() methods:

 System.out.println( s3.equals(s2) ); 

[ October 08, 2003: Message edited by: Jeff Bosch ]

[Corey McGlone]

Even though "hello" and "world" are string literals, you're using concatenation with a variable to put them together. When you do that, a brand new String is created.
However, if you don't use the variable (as is done with s4 and s5), the compiler can perform an optimization and create a single "helloworld" string literal.

[Marlene Miller]

�hello� + �world� is a constant expression.
final String s = �world�;
�hello� + s is a constant expression.
String s = �world�;
�hello� + s is not a constant expression.
This is how the concatenation operator works.
StringBuffer sb = new StringBuffer();
sb.append(�hello�);
sb.append(s);
String x = sb.toString(); // a new String object is created

   void m() {
      String s1 = "world";
      String s2 = "hello" + s1;
   }
//
Method void m()
   0 ldc #2 <String "world">
   2 astore_1
//
   3 new #3 <Class java.lang.StringBuffer>
   6 dup
   7 invokespecial #4 <Method java.lang.StringBuffer()>
// 
  10 ldc #5 <String "hello">
  12 invokevirtual #6
     <Method java.lang.StringBuffer append(java.lang.String)>
//
  15 aload_1
  16 invokevirtual #6
     <Method java.lang.StringBuffer append(java.lang.String)>
//
  19 invokevirtual #7 <Method java.lang.String toString()>
  22 astore_2
  23 return

You can see the StringBuffer method calls in the byte codes. You can see the call to toString() that creates a new String object.

[Jeff Bosch]

Hi, Marlene -
How did you get that listing?
Thanks!

[Marlene Miller]

Hi Jeff.
javap is a utility that comes with the SDK. It disassembles a class file.
javap �c classname
The �c switch prints the byte codes.
You probably know where the byte codes are defined. Java Virtual Machine Spec Chapter 6.
javap �private classname
The �private switch prints all members of a class. This is very useful for seeing the synthetic fields the compiler adds to inner classes.

[Jeff Bosch]

Thanks, Marlene!
I have a background in hardware and assembly language, so I love digging into the depths of how machines work. This'll be fun.