The difference between static methods and instance methods in java
Nov 18, 2019 pm 04:43 PM
The difference between static methods and instance methods is mainly reflected in two aspects:
When calling static methods externally, you can use "class name" .Method name" method, you can also use the "Object name.Method name" method. Instance methods only have the latter method. In other words, calling static methods does not require creating an object.
When static methods access members of this class, they are only allowed to access static members (i.e. static member variables and static methods), but are not allowed to access instance member variables and instance methods. ;Instance methods do not have this restriction.
The following examples illustrate this difference.
1. Call static method instance
public class hasStaticMethod{
//定義一個靜態(tài)方法
public static void callMe(){
System.out.println("This is a static method.");
}
}The following program uses two forms to call static methods.
public class invokeStaticMethod{
public static void main(String args[]){
hasStaticMethod.callMe(); //不創(chuàng)建對象，直接調(diào)用靜態(tài)方法
hasStaticMethod oa = new hasStaticMethod(); //創(chuàng)建一個對象
oa.callMe(); //利用對象來調(diào)用靜態(tài)方法
}
} It is allowed for the program to call static methods twice. The output of the program is as follows:
This is a static method.This is a static method.
Allowing static methods to be called without creating an object is Java's way of reducing programmer calls. It eliminates the trouble of using some common methods and allows programmers to use methods in the traditional way of using functions in C language.
2. Example of static method accessing member variables
class accessMember{
private static int sa; //定義一個靜態(tài)成員變量
private int ia; //定義一個實例成員變量
//下面定義一個靜態(tài)方法
static void statMethod(){
int i = 0; //正確，可以有自己的局部變量sa = 10;
//正確，靜態(tài)方法可以使用靜態(tài)變量
otherStat();
//正確，可以調(diào)用靜態(tài)方法
ia = 20; //錯誤，不能使用實例變量
insMethod(); //錯誤，不能調(diào)用實例方法
}
static void otherStat(){}
//下面定義一個實例方法
void insMethod(){
int i = 0; //正確，可以有自己的局部變量
sa = 15; //正確，可以使用靜態(tài)變量
ia = 30; //正確，可以使用實例變量
statMethod(); //正確，可以調(diào)用靜態(tài)方法
}
}This example can actually be summarized in one sentence: static methods can only access static members, and instance methods can access static and instance members. The reason why static methods are not allowed to access instance member variables is because instance member variables belong to a certain object, and the object does not necessarily exist when a static method is executed.
Similarly, because instance methods can access instance member variables, if a static method is allowed to call instance methods, it will indirectly be allowed to use instance member variables, so it cannot call instance methods. For the same reason, the keyword this cannot be used in static methods.
The main() method is a typical static method. It also follows the rules of general static methods, so it can be called by the system before creating the object.
Recommended tutorial: Java tutorial
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