Summary of Java Systematic Knowledge Points (1)

Summary of Java Systematic Knowledge Points (1)

  Review the knowledge points encountered when learning java in the sophomore year, and take the time to take a note, so that you can go back and learn when you forget.

	package 基础问题练习;
	
	public class 优先级1 {
    
    
	
	   public static void main(String[] args) {
    
    
	   
	        String s1 = "ABC";
	        String s2 = "ABC";
	        
	        System.out.println("s1 == s2 is:" + s1 == s2);
	        
	    }
	}

	结果：false

  Reason: In Java, the priority of the + operation method is greater than ==, so the output part of the expression is equal to "s1 == s2 is:ABC" == "runoob", and the result of the expression is false.

  As shown in the figure below, the comparison of the execution results of System.out.println("s1 == s2 is:" + s1); and System.out.println("s1 == s2 is:" + s1 == s2);:


  Reference to the same object:

		public class 优先级 2{
    
    
	    public static void main(String[] args) {
    
    
	    
	        String s1 = "ABC";
	        String s2 = "ABC";  //  s1和s2是同一个字符串
	        
	        System.out.println(s1==s2);  //语句 1
	        System.out.println("s1 == s2 is:" + s1 == s2);  // 语句2 中“+”的级别优先于“==”，所以，这句相当于"s1 == s2 is:" == s2,即判断"s1 == s2 is:ABC"=="ABC",结果为false。
	        System.out.println("s1 == s2 is:" + (s1 == s2));  // 语句 3
	        //语句1和语句3实际上是一样的。
	    }
	}

结果：
	true
	false
	s1 == s2 is:true

  Open up new objects:

		public class 优先级 2{
    
    
		public static void main(String[] args) {
    
    
		
		        String s1 = "ABC";
		        String s2 = new String("ABC");  // 新开辟一个对象，地址不同，所以对比是false。
		        
		        System.out.println(s1==s2);
		        System.out.println("s1 == s2 is:" + s1 == s2);
		        System.out.println("s1 == s2 is:" + (s1 == s2));
		        }
		}

结果：
	false
	false
	s1 == s2 is:false

  Further graphic explanations here:

  Comparison of basic data:

	public class 优先级3 {
    
    
	    public static void main(String[] args) {
    
    
	        int i = 10;
	        int k = new Integer(10);
	        int j = k;
	        System.out.println(i==j);
	        System.out.println(i==k);           //基本数据不受影响的。
	    }
	}

结果：
true
true