Strange String pool behavior

Question

I\'ve got a question of some strange String pool behavior. I\'m using == to compare equal Strings to find out whether they\'re in the pool or not.

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Answer 1

After decompiling this line

System.out.println("555" == "555" + "");

I got this bytecode

    LINENUMBER 8 L0
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    ICONST_1
    INVOKEVIRTUAL java/io/PrintStream.println(Z)V
    ...

which is equivalent to

  System.out.println(true);

that means expression "555" == "555" + "" compiles to boolean true.

For giveLiteralString() == giveLiteralString() + "" javac built this bytecode

    LINENUMBER 8 L0
    INVOKESTATIC Test1.giveLiteralString()Ljava/lang/String;
    NEW java/lang/StringBuilder
    DUP
    INVOKESTATIC Test1.giveLiteralString()Ljava/lang/String;
    INVOKESTATIC java/lang/String.valueOf(Ljava/lang/Object;)Ljava/lang/String;
    INVOKESPECIAL java/lang/StringBuilder.(Ljava/lang/String;)V
    INVOKEVIRTUAL java/lang/StringBuilder.toString()Ljava/lang/String;
    IF_ACMPNE L1
    ...

which is equivalent to

if (giveLiteralString() == new StringBuilder(giveLiteralString()).append("").toString()) {
...

which will always produce false since here we're comparing 2 disctinct objects.