StringBuilder赋值和append方法

public static void main(String[] args) {
StringBuilder a = new StringBuilder("a");
StringBuilder b = new StringBuilder("b");
append(a, b);
System.out.println(a);// ab
System.out.println(b);// b

}

public static void append(StringBuilder a, StringBuilder b) {
a.append("b");
System.out.println(a);// ab
b = a;
System.out.println(b);// ab

}
}

为什么main方法中b引用的值不是ab而是b？
append()和“=”的赋值方式有啥不同？

javac会把 str += "b"; 变成一个StringBuilder的append 操作。
相当于一个 new StringBuilder().append("a").append("b").toString().

比自己调用append费事多了。
Java code

StringBuilder builder = new StringBuilder("a");
String s = builder.append("b").append("c").append("d").toString();

String a = "a";
a += "b";
a += "c";
a += "d";

javap -c 的输出：
Java code
Code:
0: new #2 // class java/lang/StringBuilder
3: dup
4: ldc #3 // String a
6: invokespecial #4 // Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
9: astore_1
10: aload_1
11: ldc #5 // String b
13: invokevirtual #6 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
16: ldc #7 // String c
18: invokevirtual #6 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
21: ldc #8 // String d
23: invokevirtual #6 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
26: invokevirtual #9 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
29: astore_2
30: ldc #3 // String a
32: astore_3
33: new #2 // class java/lang/StringBuilder
36: dup
37: invokespecial #10 // Method java/lang/StringBuilder."<init>":()V
40: aload_3
41: invokevirtual #6 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
44: ldc #5 // String b
46: invokevirtual #6 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
49: invokevirtual #9 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;

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