java中null+““和null+null会等于什么鬼呢？

小灰灰 2022-11-06 03:59 300阅读 0赞

> **在java中，null+""会等于什么鬼?**
> 
> **null+""=?**
> 
> **null+null又会等于什么鬼呢?**
> 
> **null+null=?**

**直接上代码和截图。**

**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

![2021030801105677.png][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2N6aDUwMA_size_16_color_FFFFFF_t_70][]

**以下代码，大家也可以亲自去测试一下，大家自己多动手去写，印象会更深刻。**

public static void main(String[] args) {
        	//null只能赋值给引用数据类型的变量，不能赋值给基本数据类型的变量
    //下面一行代码语法报错，连编译都通不过，因为得益于Eclipse的强大，Eclipse会帮助我们智能的检查语法    	
    //    	int a = null; 
        	Integer b = null;
    //下面一行代码欺骗过了Eclipse的语法检查，变相的把null赋值给了基本数据类型的变量
    //int c = b;//报错NullPointerException(是在程序运行时报的错，但是编译可以通过，即编译时不会报错，运行时才会报错)
        	System.out.println(b); //null
    //    	System.out.println(c);
        	
        	Integer d = new Integer(97);
        	
    		String str1 = null;
    		System.out.println(str1);//null
    		System.out.println(str1 + "");//null
    		String str2 = null;
    		System.out.println((str2 + "").length());//4
    		String str3 = null;
    		System.out.println(str3 + 567);//null567
    		String str4 = null;
    		System.out.println((str4 + 567).length());//7
    		String str5 = null;
    		String str6 = null;
    		System.out.println(str5 == null);//true
    		System.out.println(str6 == null);//true
    		String str7 = str5 + str6;
    		System.out.println(str7);//nullnull
    		System.out.println(str7 == null);//false
    		System.out.println(str7.length());//8
    		String str8 = null;
    		System.out.println(str8 + 666);//null666
    		System.out.println(str8 + 999.88);//null999.88
    		System.out.println(str8 + b);//nullnull
    		System.out.println(str8 + true);//nulltrue
    		System.out.println(str8 + false);//nullfalse
    		System.out.println(str8 + d);//null97
    		System.out.println(str8 + 'a');//nulla
    		System.out.println("**************");
    		String s1 = null;
    		System.out.println("".equals(s1));//false
    		//报错java.lang.NullPointerException
    //		System.out.println(s1.equals(""));
    		System.out.println("**************");
    		String s2 = null;
    		System.out.println("null".equals(s2));//false
    		System.out.println("".equals(s2));//false
    		System.out.println("null".equals((String)s2));//false
    		System.out.println(((String)s2) == null);//true
    		System.out.println(s2 == null);//true
    		System.out.println("**************");
    		String s3 = "";
    		String s4 = "null";
    		System.out.println(s3.equals(null));//false
    		System.out.println(s3.equals((String)null));//false
    		System.out.println(s4.equals(null));//false
    		System.out.println(s4.equals((String)null));//false
    		System.out.println("".equals(null));//false
    		System.out.println("".equals((String)null));//false
    		String str9 = "江西省赣州市于都县";
    		System.out.println(str9 + 6666);//江西省赣州市于都县6666
    		System.out.println("==============");
    		String s5 = null;
    		String s6 = null;
    		System.out.println(s5 == s6);//true
    		System.out.println(((String)s5) == ((String)s6));//true
    		System.out.println("##################");
    //		System.out.println(null); //语法报错
    		System.out.println((String)null);//null
    //下面一行代码会报异常，java.lang.NullPointerException
    		System.out.println(((String)null).length());
    	}

**运行结果如下：**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2N6aDUwMA_size_16_color_FFFFFF_t_70 1][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2N6aDUwMA_size_16_color_FFFFFF_t_70 2][]

[2021030801105677.png]: /images/20221023/69b2d10cf89d469cac6959bdbc4c4363.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2N6aDUwMA_size_16_color_FFFFFF_t_70]: /images/20221023/96ce56b161444681ae9b0d0875a0ee3d.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2N6aDUwMA_size_16_color_FFFFFF_t_70 1]: /images/20221023/1613aab727284c7989a4d97f4957adb0.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2N6aDUwMA_size_16_color_FFFFFF_t_70 2]: /images/20221023/ed8fa1ef68bd4389aec9bdc37ce12df0.png