JDK1.8字符串拼接的底层实现

public class A {
	public static void main(String[] args) {
		String a = "abc";
		a = a + "d";
	}
} 

编译器编译之后的等效代码

public class A {
	public static void main(String[] args) {
		String a = "abc";
		StringBuilder temp = new StringBuilder(a);
		temp.append("d");
		a = temp.toString();
	}
}