obj == null vs null == obj

Question

I have always used to check for null like

if(null==obj)

When I compiled my code and looked into .class file after decompiling, I could see that my code got changed to

if(obj==null)

I know in java null==obj and obj==nulldoesn't matter. But I'm curious to know why compiler changed it?

Answer 1

The compiler did not change anything. It faithfully compiled if (null == obj) and if (obj == null) into different bytecodes, which decompilers converted back to the same Java code.

Comparison with null on the right, i.e.

if (o == null) {
    ...
}

gets translated to this byte code with ifnonnull instruction:

0: aload_0
1: ifnonnull     ...

Comparison with null on the left, i.e.

if (null == o) {
    ...
}

gets translated to a different bytecode with if_acmpne instruction:

0: aconst_null
1: aload_0
2: if_acmpne     ...

In theory, decompiler has enough information to figure out which way the arguments are ordered in the source file. However, they produced the same code for both orderings.