A brief discussion on the difference between equals and ==

When you first learn Java, you may often encounter the following code:

1 String str1 = new String("hello");

2 String str2 = new String("hello");

3    

4 System.out.println(str1==str2);

5 System.out.println (str1.equals(str2));

Why are the output results of lines 4 and 5 different? What is the difference between == and equals methods? If you don't figure this out when you first learn Java, you will make some low-level errors when writing code in the future. Today let’s learn about the difference between == and equals methods.

1. What exactly does the relational operator "==" compare?

The following sentence is taken from the original words in the book "Java Programming Thoughts":

"Relationship Operators produce a boolean result, and they compute a relationship between the values ​​of their operands."

This sentence seems simple, but it still needs to be understood carefully. To put it simply, == is used to compare whether values ​​are equal. Let’s look at a few examples:

public class Main {

/**

     * @param args

     */

public static void main(String[] args) {

// TODO Auto-generated method stub

int n=3;

int m=3;

System.out.println(n==m);

String str = new String("hello" );

String str1 = new String("hello");

String str2 = new String("hello");

System. out.println(str1==str2);

str1 = str;

str2 = str;

System.out.println(str1= =str2);

}

}

The output result is true false true

n==m The result is true. This is easy to understand. The values ​​stored in variable n and variable m are both 3, which must be equal. And why are the results of the two comparisons of str1 and str2 different? To understand this, you only need to understand the difference between basic data type variables and non-basic data type variables.

There are 8 basic data types in Java:

• Floating point type: float (4 byte), double(8 byte)

• Integer type: byte(1 byte), short(2 byte), int (4 byte) , long(8 byte)

• Character type: char(2 byte)

• Boolean: boolean (The JVM specification does not clearly stipulate the size of the space it occupies, but only stipulates that it can only take the literal values ​​"true" and "false")

For variables of these 8 basic data types, the variables directly store "value", so when comparing using the relational operator ==, compare is the "value" itself. It should be noted that floating point and integer types are both signed types, while char is an unsigned type (the value range of the char type is 0~2^16-1).

That is to say, for example:

int n=3;

int m=3;

Variable n and variable m both directly store the value "3", so when compared with ==, the result is true.

For variables of non-basic data types, they are called reference type variables in some books. For example, str1 above is a reference type variable. A reference type variable stores not the "value" itself, but the address of its associated object in memory. For example, the following line of code:

String str1;

This sentence declares a reference type variable, It is not associated with any object at this time.

Use new String("hello") to generate an object (also called an instance of the String class) and bind this object to str1:

str1= new String("hello");

Then str1 points to an object (str1 is also called in many places Object reference), at this time, the variable str1 stores the storage address of the object it points to in the memory, not the "value" itself, that is to say, it is not the directly stored string "hello". The references here are very similar to pointers in C/C++.

So when you use == to compare str1 and str2 for the first time, the result is false. Therefore, they point to different objects respectively, which means that the memory addresses where they are actually stored are different.

In the second comparison, both str1 and str2 point to the object pointed to by str, so the result is undoubtedly true.

2. What does equals compare?

The equals method is a method in the base class Object, so all classes that inherit from Object will have this method. In order to understand the role of the equals method more intuitively, let's look directly at the implementation of the equals method in the Object class.

The source code path of this class is: Object.java under the java.lang path of src.zip in C:\Program Files\Java\jdk1.6.0_14 (depending on personal jdk installation depending on the path).

The following is the implementation of the equals method in the Object class:

Very Obviously, in the Object class, the equals method is used to compare whether the references of two objects are equal, that is, whether they point to the same object.

But some friends may have questions again, why is the output result of the following code true?

public class Main {

/**

     * @param args

     */

public static void main( String[] args) {

// TODO Auto-generated method stub

String str1 = new String("hello");

String str2 = new String("hello");

System.out.println(str1.equals(str2));

}

}

To know the truth, you can look at the specific implementation of the equals method of the String class. Also under this path, String.java is the implementation of the String class.

The following is the specific implementation of the equals method in the String class:

It can be seen that the String class overrides the equals method to compare whether the strings stored in the pointed string objects are equal.

Some other classes, such as Double, Date, Integer, etc., have rewritten the equals method to compare whether the contents stored in the pointed objects are equal.

In summary:

1) For ==, if it acts on a variable of a basic data type, its stored " Value" are equal;

If it is applied to a reference type variable, the comparison is the address of the object pointed to

2) For equals method, note: the equals method cannot act on variables of basic data types

If the equals method is not overridden, the address of the object pointed to by the reference type variable is compared. ;

If classes such as String and Date override the equals method, the contents of the pointed objects will be compared.

The above is the detailed content of A brief discussion on the difference between equals and ==. For more information, please follow other related articles on the PHP Chinese website!