Java syntax ---- difference between equals and = in Java

Java syntax ---- difference between equals and = in Java
[Body] When I was learning the Android and Java languages, I always met the two characters "equals" and "=", which I always felt similar. In fact, there are still some differences, today, I want to thoroughly understand them. 1. Meaning of the Data Type and "=" in java: Basic Data Type (also known as raw data type): byte, short, char, int, long, float, double, boolean. The comparison between them applies the double equal sign (=) to compare their values. Composite data types (classes): when they use (=) for comparison, they compare the storage addresses in the memory (specifically, the heap memory address ). NOTE: For the second type, the comparison result is true unless it is a new object. Otherwise, the comparison result is false. Because every new time, heap memory space will be re-opened. Ii. equals () method Introduction: All classes in JAVA are inherited from the super class of Object, and an equals method is defined in the Object class, the initial behavior of this method is to compare the memory address of the object. However, in some class libraries, this method is rewritten, such as String, Integer, and Date. Among these classes, equals has its own implementation, instead of comparing the storage addresses of classes in the heap memory. Therefore, for equals comparison between composite data types, if the equals method is not overwritten, the comparison between them is the address value of the storage location in the memory, the result is the same as that of the dual equal sign (=). If the result is rewritten, the result is based on the requirements of the write operation. Iii. equals () method of the String class: Now let's take the String class as an example: we find the String class in the \ src \ java \ lang directory and find that the equals method is rewritten as follows: copy code 1 public boolean equals (Object anObject) {2 if (this = anObject) {3 return true; 4} 5 if (anObject instanceof String) {6 String anotherString = (String) anObject; 7 int n = value. length; 8 if (n = anotherString. value. length) {9 char v1 [] = value; 10 char v2 [] = anotherString. value; 11 int I = 0; 12 while (N --! = 0) {13 if (v1 [I]! = V2 [I]) 14 return false; 15 I ++; 16} 17 return true; 18} 19} 20 return false; 21} copy the above Code to see that, the rewritten equals () method in the String class is actually comparing the content of two strings. Next we will look at the comparison of the String class through the actual code. 1. Example code: Copy code 1 public class StringDemo {2 public static void main (String [] args) {3 String s1 = "Hello "; 4 String s2 = "Hello"; 5 System. out. println (s1 = s2); // true6} 7} copy the code above and use "=" to compare s1 and s2. The returned result is true. 2. If you change the program a little, you may find that: Copy code 1 public class StringDemo {2 public static void main (String args []) {3 String str1 = "Hello "; 4 String str2 = new String ("Hello"); 5 String str3 = str2; // pass 6 System for reference. out. println (str1 = str2); // false 7 System. out. println (str1 = str3); // false 8 System. out. println (str2 = str3); // true 9 System. out. println (str1.equals (str2); // true10 System. out. println (str1.equ Als (str3); // true11 System. out. println (str2.equals (str3); // true12} 13} copy the code above the Code. In the code above the 4th line, we create an object and use "=" to compare s1 and s2, the returned result is false. If "equals" is used to compare s1 and s2, the returned result is true. To analyze the above Code, we must first analyze the heap memory space and stack memory space. This is very important: after reading the figure above and then combining the above code, we can see it at a glance. Now we can give ourselves an interview question: What is the difference between "=" and "equals ()" in the string comparison? =: Compare whether the values of the two strings memory addresses are equal, which is a numerical comparison; equals (): Compare the content of the two strings, which is a content comparison. Equals () will be used for subsequent equality Determination of strings (). 3. Change the program again: Copy code 1 public class ObjectDemo {2 public static void main (String [] args) {3 String s1 = "Hello "; 4 String s2 = new String ("Hello"); 5 s2 = s2.intern (); 6 System. out. println (s1 = s2); // true7 System. out. println (s1.equals (s2); // true8} 9} copy the code in the 5th rows of the above Code, java. lang. the intern () method of String "abc ". the Return Value of the intern () method is still the string "abc". On the surface, it seems that this method is useless. But in fact, it performs a small action: Check whether there is a string such as "abc" in the string pool. If yes, the string in the pool is returned; if not, this method adds "abc" to the string pool and then returns its reference. 4. Compare the values of two objects: the code is as follows: Copy code 1 public class ObjectDemo {2 public static void main (String args []) {3 Student student1 = new Student ("Life 1", 22, "Chengdu"); 4 Student student2 = new Student ("Life 1", 22, "Chengdu"); 5 System. out. println (student1 = student2); 6 System. out. println (student1.equals (student2); 7} 8} 9 class Student {10 private String name; 11 private int age; 12 private String address; 13 public Student (Strin G name, int age, String address) {14 this. name = name; 15 this. age = age; 16 this. address = address; 17} 18 // rewrite the equals method in the Object class (compare the values of the two objects) 19 public boolean equals (Object obj) {20 // if the memory address is equal, it must be the same object, so you do not need to compare the attribute values of the two objects 21 if (this = obj) {22 return true; 23} 24 // determine whether obj is a Baboon instance 25 if (obj instanceof Student) {26 Student B = (Student) obj; // force convert 27 // determine whether the attribute values of the two objects are equal 28 if (! This. name. equals (B. name) {29 return false; 30} 31 else if (this. age! = B. age) {32 return false; 33} 34 else if (this. address! = B. address) {35 return false; 36} 37 return true; 38} else {39 return false; 40} 41} 42} copy the code in the above Code, first, determine whether the passed object is equal to the address of the current object. If it is equal, it must be the same object. Because the passed parameters are of the Object type, any Object can receive them. Once it is received, the Object type will be transformed down and then judged.