java中使用==比较String字符串

直接上代码和截图

public static void main(String[] args) {
		String str1 = "abc";
		String str2 = "abc";
		String str3 = "efg";
		String str4 = new String("abc");
		String str5 = new String("abc");
		String str6 = new String("efg");
		String str7 = "efg";
		System.out.println(str1 == str2);//true
		System.out.println(str1 == str4);//false
		System.out.println(str1 == str5);//false
		System.out.println(str4 == str5);//false
		System.out.println(str3 == str6);//false
		System.out.println(str3 == str7);//true
		System.out.println(str1 == str3);//false
	}

运行结果如下：

注意：看如下代码，看看null+null等于什么？

System.out.println(null + null);//报错,连编译都通不过
		
		String n = null;
		String m = null;
		System.out.println(n + m);//nullnull