A.class

Compiled from "A.java"
public class A {
  public A();
    Code:
       0: aload_0       
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return        

  public static void main(java.lang.String[]);
    Code:
      // String s = null;
       0: aconst_null   
       1: astore_1      
      // s += "A";
       2: new           #2                  // class java/lang/StringBuilder
       5: dup           
       6: invokespecial #3                  // Method java/lang/StringBuilder."<init>":()V
       9: aload_1       
      // 型がStringなのでそっちを呼ぶ
      // See also: http://docs.oracle.com/javase/6/docs/api/java/lang/StringBuilder.html#append(java.lang.String)
      10: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      13: ldc           #5                  // String A
      15: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      18: invokevirtual #6                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      // s = null + "A"
      21: astore_1      
      22: new           #2                  // class java/lang/StringBuilder
      25: dup           
      26: invokespecial #3                  // Method java/lang/StringBuilder."<init>":()V
      29: aconst_null   
      // ただのnullはObjectで呼ぶ
      // See also: http://docs.oracle.com/javase/6/docs/api/java/lang/StringBuilder.html#append(java.lang.Object)
      30: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/Object;)Ljava/lang/StringBuilder;
      33: ldc           #5                  // String A
      35: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      38: invokevirtual #6                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      41: astore_1      
      42: return        
}

A.java

public class A {
    public static void main(String[] args) {
        String s = null;
        s += "A";
        s = null + "A";
    }
}