On Java String Concatenation

Most Java programmers know to use a StringBuffer (or a JDK 1.5 StringBuilder) when concatenating Strings in Java,
but it occurred to me that the compiler might be able to optimize some of these calls.

For instance, look at the following code:

    StringBuffer buf = new StringBuffer();
    buf.append("one" + "two");    


There is an obvious optimization there: combine “one” and “two” into a single String before appending them to the
StringBuffer.

Sure enough:

   L0 (0)
    NEW StringBuffer
    DUP
    INVOKESPECIAL StringBuffer.() : void
    ASTORE 0: buf
   L1 (5)
    ALOAD 0: buf
    LDC "onetwo"
    INVOKEVIRTUAL StringBuffer.append(String) : StringBuffer
    POP

Note the LDC "onetwo" line – the compiler has combined the two strings in the bytecode itself.

However, the following code:

    String one = "str1";
    StringBuffer buf = new StringBuffer();
    buf.append(one + "two");    


gives:

   L0 (0)
    LDC "str1"
    ASTORE 0: one
   L1 (3)
    NEW StringBuffer
    DUP
    INVOKESPECIAL StringBuffer.() : void
    ASTORE 1: buf
   L2 (8)
    ALOAD 1: buf
    NEW StringBuffer
    DUP
    ALOAD 0: one
    INVOKESTATIC String.valueOf(Object) : String
    INVOKESPECIAL StringBuffer.(String) : void
    LDC "two"
    INVOKEVIRTUAL StringBuffer.append(String) : StringBuffer
    INVOKEVIRTUAL StringBuffer.toString() : String
    INVOKEVIRTUAL StringBuffer.append(String) : StringBuffer
    POP


Hmm – that isn't good at all. The code one + "two" causes the compiler to create a new StringBuffer, concatenate
the two Strings, call toString on the temporary StringBuffer and append that to the original StringBuffer. Looks like that
should be written:

    String one = "str1";
    StringBuffer buf = new StringBuffer();
    buf.append(one);
    buf.append("two");   


which gives:

   L0 (0)
    LDC "str1"
    ASTORE 0: one
   L1 (3)
    NEW StringBuffer
    DUP
    INVOKESPECIAL StringBuffer.() : void
    ASTORE 1: buf
   L2 (8)
    ALOAD 1: buf
    ALOAD 0: one
    INVOKEVIRTUAL StringBuffer.append(String) : StringBuffer
    POP
   L3 (13)
    ALOAD 1: buf
    LDC "two"
    INVOKEVIRTUAL StringBuffer.append(String) : StringBuffer
    POP


Much better!

There is one final (pun intended) change that I found interesting:

    final String one = "str1";
    StringBuffer buf = new StringBuffer();
    buf.append(one + "two");


gives:

   L0 (0)
    LDC "str1"
    ASTORE 0: one
   L1 (3)
    NEW StringBuffer
    DUP
    INVOKESPECIAL StringBuffer.() : void
    ASTORE 1: buf
   L2 (8)
    ALOAD 1: buf
    LDC "str1two"
    INVOKEVIRTUAL StringBuffer.append(String) : StringBuffer
    POP


Nice!

Obviously most of this is pretty logical and well known by most Java programmers. I found it interesting to see exactly what
optimizations the complier (as opposed to the JVM) is doing though. (Note that these experiments were done using the
Eclipse 3.0 JDK 1.4 complier. Other compiler optimizations may vary)

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