类中的“静态”关键字有什么作用？

Question

To be specific, I was trying this code:

package hello;

public class Hello {

    Clock clock = new Clock();

    public static void main(String args[]) {
        clock.sayTime();
    }
}

But it gave the error

Cannot access non-static field in static method main

So I changed the declaration of clock to this:

static Clock clock = new Clock();

And it worked. What does it mean to put that keyword before the declaration? What exactly will it do and/or restrict in terms of what can be done to that object?

Answer 1

static members belong to the class instead of a specific instance.

It means that ^[1] even if you create a million instances of the class or you don't create any. It will be shared by all instances.

Since static methods also do not belong to a specific instance, they can't refer to instance members. In the example given, main does not know which instance of the Hello class (and therefore which instance of the Clock class) it should refer to. static members can only refer to static members. Instance members can, of course access static members.

Side note: Of course, static members can access instance members .

Example:

public class Example {
    private static boolean staticField;
    private boolean instanceField;
    public static void main(String[] args) {
        // a static method can access static fields
        staticField = true;

        // a static method can access instance fields through an object reference
        Example instance = new Example();
        instance.instanceField = true;
    }

[1]: Depending on the runtime characteristics, it can be one per ClassLoader or AppDomain or thread, but that is beside the point.

Answer 2

It means that there is only one instance of "clock" in Hello, not one per each separate instance of the "Hello" class, or more-so, it means that there will be one commonly shared "clock" reference among all instances of the "Hello" class.

So if you were to do a "new Hello" anywhere in your code: A- in the first scenario (before the change, without using "static"), it would make a new clock every time a "new Hello" is called, but B- in the second scenario (after the change, using "static"), every "new Hello" instance would still share and use the initial and same "clock" reference first created.

Unless you needed "clock" somewhere outside of main, this would work just as well:

package hello;
public class Hello
{
    public static void main(String args[])
    {
      Clock clock=new Clock();
      clock.sayTime();    
    }
}

Answer 3

The static keyword means that something (a field, method or nested class) is related to the type rather than any particular instance of the type. So for example, one calls Math.sin(...) without any instance of the Math class, and indeed you can't create an instance of the Math class.

For more information, see the relevant bit of Oracle's Java Tutorial .

---

Sidenote

Java unfortunately allows you to access static members as if they were instance members, eg

// Bad code!
Thread.currentThread().sleep(5000);
someOtherThread.sleep(5000);

That makes it look as if sleep is an instance method, but it's actually a static method - it always makes the current thread sleep. It's better practice to make this clear in the calling code:

// Clearer
Thread.sleep(5000);

Answer 4

The static keyword in Java means that the variable or function is shared between all instances of that class as it belongs to the type , not the actual objects themselves.

So if you have a variable: private static int i = 0; and you increment it ( i++ ) in one instance, the change will be reflected in all instances. i will now be 1 in all instances.

Static methods can be used without instantiating an object.

Answer 5

Basic usage of static members...

public class Hello
{
    // value / method
    public static String staticValue;
    public String nonStaticValue;
}

class A
{
    Hello hello = new Hello();
    hello.staticValue = "abc";
    hello.nonStaticValue = "xyz";
}

class B
{
    Hello hello2 = new Hello(); // here staticValue = "abc"
    hello2.staticValue; // will have value of "abc"
    hello2.nonStaticValue; // will have value of null
}

That's how you can have values shared in all class members without sending class instance Hello to other class. And whit static you don't need to create class instance.

Hello hello = new Hello();
hello.staticValue = "abc";

You can just call static values or methods by class name:

Hello.staticValue = "abc";

Answer 6

Static means that you don't have to create an instance of the class to use the methods or variables associated with the class. In your example, you could call:

Hello.main(new String[]()) //main(...) is declared as a static function in the Hello class

directly, instead of:

Hello h = new Hello();
h.main(new String[]()); //main(...) is a non-static function linked with the "h" variable

From inside a static method (which belongs to a class) you cannot access any members which are not static, since their values depend on your instantiation of the class. A non-static Clock object, which is an instance member, would have a different value/reference for each instance of your Hello class, and therefore you could not access it from the static portion of the class.

Answer 7

Static in Java:

Static is a Non Access Modifier. The static keyword belongs to the class than instance of the class. can be used to attach a Variable or Method to a Class.

Static keyword CAN be used with:

Method

Variable

Class nested within another Class

Initialization Block

CAN'T be used with:

Class (Not Nested)

Constructor

Interfaces

Method Local Inner Class(Difference then nested class)

Inner Class methods

Instance Variables

Local Variables

Example:

Imagine the following example which has an instance variable named count which in incremented in the constructor:

package pkg;

class StaticExample {
    int count = 0;// will get memory when instance is created

    StaticExample() {
        count++;
        System.out.println(count);
    }

    public static void main(String args[]) {

        StaticExample c1 = new StaticExample();
        StaticExample c2 = new StaticExample();
        StaticExample c3 = new StaticExample();

    }
}

Output:

1 1 1

Since instance variable gets the memory at the time of object creation, each object will have the copy of the instance variable, if it is incremented, it won't reflect to other objects.

Now if we change the instance variable count to a static one then the program will produce different output:

package pkg;

class StaticExample {
    static int count = 0;// will get memory when instance is created

    StaticExample() {
        count++;
        System.out.println(count);
    }

    public static void main(String args[]) {

        StaticExample c1 = new StaticExample();
        StaticExample c2 = new StaticExample();
        StaticExample c3 = new StaticExample();

    }
}

Output:

1 2 3

In this case static variable will get the memory only once, if any object changes the value of the static variable, it will retain its value.

Static with Final:

The global variable which is declared as final and static remains unchanged for the whole execution. Because, Static members are stored in the class memory and they are loaded only once in the whole execution. They are common to all objects of the class. If you declare static variables as final, any of the objects can't change their value as it is final. Therefore, variables declared as final and static are sometimes referred to as Constants. All fields of interfaces are referred as constants, because they are final and static by default.

Picture Resource : Final Static

Answer 8

To add to existing answers, let me try with a picture:

An interest rate of 2% is applied to ALL savings accounts. Hence it is static .

A balance should be individual , so it is not static.

Answer 9

This discussion has so far ignored classloader considerations. Strictly speaking, Java static fields are shared between all instances of a class for a given classloader .

Answer 10

The keyword static is used to denote a field or a method as belonging to the class itself and not the instance. Using your code, if the object Clock is static, all of the instances of the Hello class will share this Clock data member (field) in common. If you make it non-static, each individual instance of Hello can have a unique Clock field.

You added a main method to your class Hello so that you could run the code. The problem with that is that the main method is static and as such, it cannot refer to non-static fields or methods inside of it. You can resolve this in two ways:

1. Make all fields and methods of the Hello class static so that they could be referred to inside the main method. This is really not a good thing to do (or the wrong reason to make a field and/or a method static)
2. Create an instance of your Hello class inside the main method and access all it's fields and methods the way they were intended to in the first place.

For you, this means the following change to your code:

package hello;

public class Hello {

    private Clock clock = new Clock();

    public Clock getClock() {
        return clock;
    }

    public static void main(String args[]) {
        Hello hello = new Hello();
        hello.getClock().sayTime();
    }
}

Answer 11

A field can be assigned to either the class or an instance of a class. By default fields are instance variables. By using static the field becomes a class variable, thus there is one and only one clock . If you make a changes in one place, it's visible everywhere. Instance varables are changed independently of one another.

Answer 12

In Java, the static keyword can be simply regarded as indicating the following:

"without regard or relationship to any particular instance"

If you think of static in this way, it becomes easier to understand its use in the various contexts in which it is encountered:

• A static field is a field that belongs to the class rather than to any particular instance

• A static method is a method that has no notion of this ; it is defined on the class and doesn't know about any particular instance of that class unless a reference is passed to it

• A static member class is a nested class without any notion or knowledge of an instance of its enclosing class (unless a reference to an enclosing class instance is passed to it)

Answer 13

I have developed a liking for static methods (only, if possible) in "helper" classes.

The calling class need not create another member (instance) variable of the helper class. You just call the methods of the helper class. Also the helper class is improved because you no longer need a constructor, and you need no member (instance) variables.

There are probably other advantages.

Answer 14

Static makes the clock member a class member instead of an instance member. Without the static keyword you would need to create an instance of the Hello class (which has a clock member variable) - eg

Hello hello = new Hello();
hello.clock.sayTime();

Answer 15

静态方法不使用定义它们的类的任何实例变量。可以在此页面上找到对差异的很好解释

Answer 16

//Here is an example 

public class StaticClass 
{
    static int version;
    public void printVersion() {
         System.out.println(version);
    }
}

public class MainClass 
{
    public static void main(String args[]) {  
        StaticClass staticVar1 = new StaticClass();
        staticVar1.version = 10;
        staticVar1.printVersion() // Output 10

        StaticClass staticVar2 = new StaticClass();
        staticVar2.printVersion() // Output 10
        staticVar2.version = 20;
        staticVar2.printVersion() // Output 20
        staticVar1.printVersion() // Output 20
    }
}

Answer 17

Can also think of static members not having a "this" pointer. They are shared among all instances.

Answer 18

Understanding Static concepts

public class StaticPractise1 {
    public static void main(String[] args) {
        StaticPractise2 staticPractise2 = new StaticPractise2();
        staticPractise2.printUddhav(); //true
        StaticPractise2.printUddhav(); /* false, because printUddhav() is although inside StaticPractise2, but it is where exactly depends on PC program counter on runtime. */

        StaticPractise2.printUddhavsStatic1(); //true
        staticPractise2.printUddhavsStatic1(); /*false, because, when staticPractise2 is blueprinted, it tracks everything other than static  things and it organizes in its own heap. So, class static methods, object can't reference */

    }
}

Second Class

public class StaticPractise2 {
    public static void printUddhavsStatic1() {
        System.out.println("Uddhav");
    }

    public void printUddhav() {
        System.out.println("Uddhav");
    }
}

Answer 19

main() is a static method which has two fundamental restrictions:

1. The static method cannot use a non-static data member or directly call non-static method.

2. this() and super() cannot be used in static context.

    class A { int a = 40; //non static public static void main(String args[]) { System.out.println(a); } }

Output: Compile Time Error

Answer 20

Static Variables Can only be accessed only in static methods, so when we declare the static variables those getter and setter methods will be static methods

static methods is a class level we can access using class name

The following is example for Static Variables Getters And Setters:

public class Static 
{

    private static String owner;
    private static int rent;
    private String car;
    public String getCar() {
        return car;
    }
    public void setCar(String car) {
        this.car = car;
    }
    public static int getRent() {
        return rent;
    }
    public static void setRent(int rent) {
        Static.rent = rent;
    }
    public static String getOwner() {
        return owner;
    }

    public static void setOwner(String owner) {
        Static.owner = owner;
    }

}

Answer 21

A question was asked here about the choice of the word 'static' for this concept. It was dup'd to this question, but I don't think the etymology has been clearly addressed. So...

---

It's due to keyword reuse, starting with C.

Consider data declarations in C (inside a function body):

    void f() {
        int foo = 1;
        static int bar = 2;
         :
    }

The variable foo is created on the stack when the function is entered (and destroyed when the function terminates). By contrast, bar is always there, so it's 'static' in the sense of common English - it's not going anywhere.

Java, and similar languages, have the same concept for data. Data can either be allocated per instance of the class (per object) or once for the entire class. Since Java aims to have familiar syntax for C/C++ programmers, the 'static' keyword is appropriate here.

    class C {
        int foo = 1;
        static int bar = 2;
         :
    }

Lastly, we come to methods.

    class C {
        int foo() { ... }
        static int bar() { ... }
         :
    }

There is, conceptually speaking, an instance of foo() for every instance of class C. There is only one instance of bar() for the entire class C. This is parallel to the case we discussed for data, and therefore using 'static' is again a sensible choice, especially if you don't want to add more reserved keywords to your language.

Answer 22

When run some project firstly load static things(variables, methods, blocks..).

When run this project main method load first. Because its static method . Then it look object "a" object .But object a not define yet. Because its non static. Then come like this error.

Answer 23

A member in a Java program can be declared as static using the keyword “static” preceding its declaration/definition. When a member is declared static, then it essentially means that the member is shared by all the instances of a class without making copies of per instance.

Thus static is a non-class modifier used in Java and can be applied to the following members:

• Variables
• Methods
• Blocks
• Classes (more specifically, nested classes)

When a member is declared static, then it can be accessed without using an object. This means that before a class is instantiated, the static member is active and accessible. Unlike other non-static class members that cease to exist when the object of the class goes out of scope, the static member is still obviously active.

Static Variable in Java

A member variable of a class that is declared as static is called the Static Variable. It is also called as the “Class variable”. Once the variable is declared as static, memory is allocated only once and not every time when a class is instantiated. Hence you can access the static variable without a reference to an object.

The following Java program depicts the usage of Static variables:

class Main
{
// static variables a and b
static int a = 10;
static int b;

static void printStatic()
{
    a = a /2;
    b = a;

    System.out.println("printStatic::Value of a : "+a + " Value of b : 
 "+b);
}  

public static void main(String[] args)
{
   printStatic();
   b = a*5;
   a++;

System.out.println("main::Value of a : "+a + " Value of b : "+b);
   }
 }

output::

printStatic::Value of a : Value of b : 5
main::Value of a : 6 Value of b : 25

In the above program, we have two static variables ie a and b. We modify these variables in a function “printStatic” as well as in “main”. Note that the values of these static variables are preserved across the functions even when the scope of the function ends. The output shows the values of variables in two functions.

Static Method

A method in Java is static when it is preceded by the keyword “static”.

Some points that you need to remember about the static method include:

• A static method belongs to the class as against other non-static methods that are invoked using the instance of a class.
• To invoke a static method, you don't need a class object.
• The static data members of the class are accessible to the static method. The static method can even change the values of the static data member.
• A static method cannot have a reference to 'this' or 'super' members. Even if a static method tries to refer them, it will be a compiler error.
• Just like static data, the static method can also call other static methods. A static method cannot refer to non-static data members or variables and cannot call non-static methods too.

The following program shows the implementation of the static method in Java:

class Main
{
  // static method
  static void static_method()
{
    System.out.println("Static method in Java...called without any 
object");
}

public static void main(String[] args)
{
    static_method();
   }
 }

output:

Static method in Java...called without any object

Static Block In Java

Just as you have function blocks in programming languages like C++, C#, etc. in Java also, there is a special block called “static” block that usually includes a block of code related to static data.

This static block is executed at the moment when the first object of the class is created (precisely at the time of classloading) or when the static member inside the block is used.

The following program shows the usage of a static block.

class Main
{
  static int sum = 0;
  static int val1 = 5;
  static int val2;

// static block
 static {
    sum = val1 + val2;
    System.out.println("In static block, val1: " + val1  + " val2: "+ 
val2 + " sum:" + sum);
    val2 = val1 * 3;
    sum = val1 + val2;
}

 public static void main(String[] args)
{
    System.out.println("In main function, val1: " + val1  + " val2: "+ val2 + " sum:" + sum);
  }
}

output:

In static block, val1: 5 val2: 0 sum:5
In main function, val1: val2: 15 sum:20

Static Class

In Java, you have static blocks, static methods, and even static variables. Hence it's obvious that you can also have static classes. In Java, it is possible to have a class inside another class and this is called a Nested class. The class that encloses the nested class is called the Outer class.

In Java, although you can declare a nested class as Static it is not possible to have the outer class as Static.

Let's now explore the static nested classes in Java.

Static Nested Class

As already mentioned, you can have a nested class in Java declared as static. The static nested class differs from the non-static nested class(inner class) in certain aspects as listed below.

Unlike the non-static nested class, the nested static class doesn't need an outer class reference.

A static nested class can access only static members of the outer class as against the non-static classes that can access static as well as non-static members of the outer class.

An example of a static nested class is given below.

---

class Main{
  private static String str = "SoftwareTestingHelp";

     //Static nested class
     static class NestedClass{
        //non-static method
            public void display() {

            System.out.println("Static string in OuterClass: " + str);
            }

    }
   public static void main(String args[])
   {
            Main.NestedClassobj = new Main.NestedClass();
            obj.display();
   }
}

output

Static string in OuterClass: SoftwareTestingHelp

I think this is how static keyword works in java.