Explain the differences between equals and = in java

First look at a piece of code:

String test1 = "add";String test2 = "add";String test3 = new String("add");String test4 = new String("add");System.out.println(test1 == test2);System.out.println(test1.equals(test2));System.out.println(test3 == test4);System.out.println(test3.equals(test4));System.out.println(test1 == test3);System.out.println(test1.equals(test4));

Running result:

truetruefalsetruefalsetrue

Analysis:

In combination with the preceding statement, I will briefly describe the value assignment process of the string. When the computer executes Stringtest1 = "add", it first checks whether there is an "add" address space in the memory. If yes, it directly points the variable test1 to the address space. If it does not exist, create an address space and store "add". Then, point the variable test1 to the address space. Therefore, test1 and test2 actually point to the same address space. When the computer executes Stringtest3 = new String ("add"), it directly opens an address space in the memory to store "add", and then points the variable test3 to the address space. The final memory distribution is shown in:

Conclusion:

From the running result and view of the program, we can conclude that "=" compares the storage addresses of two variables. As long as the addresses are different, they are not equal. The "equals" requirement is not so harsh. If the value is equal, it is true.

 

Further dive:

Let's take a look at the source code of the equals method of the object String:

public boolean equals(Object anObject) { if (this == anObject) {     return true; } if (anObject instanceof String) {     String anotherString = (String)anObject;     int n = count;     if (n == anotherString.count) {  char v1[] = value;  char v2[] = anotherString.value;  int i = offset;  int j = anotherString.offset;  while (n-- != 0) {    if (v1[i++] != v2[j++])      return false;  }     return true;  } } return false;}

From the source code, we can see that the String class overrides the equals method of the Object class. It also uses "=" to compare the memory address, but the String class extends the comparison, the result is different from "=.

 

Extension:

"=" Is the equal implementation of the base class Object. Different classes rewrite it differently, resulting in different comparisons. Therefore, you can create a new class, and then rewrite the equals method of the base class to compare it in your own way. Of course, the equals method is rarely rewritten. We may be more likely to use the toString method, but their principles are the same.