In the same project I have two packages, 1st package contains a class with this code:
package com.ginger;
public class SimplePrint
{
public SimplePrint(){}
public static void print(Object obj)
{
System.out.println(obj);
}
}
I would like use the method print() in another class in another package, but within the same project.
import com.ginger.*;
public class MainClass
{
public static void main(String[] args)
{
print("Some");
}
}
But the compiler tells me that the method print() is undefined for the 2nd class.
In the same time, I am able to create the object SimplePrint s = new SimplePrint() in the 2nd class.
I'm new to the programming, excuse me if i am asking about the simple thing.
There are a couple of ways to do this:
Non-static
Remove the static keyword of the method print and create an instance of the class
SimplePrint simplePrint = new SimplePrint();
and just do this
simplePrint.print("");
Or combine the above into a single line:
new SimplePrint().print("");
Static
You keep the print method static and just do this
SimplePrint.print("");
static keyword for a method implies its a class level.
If keyword like static is not used for a method it means instance level,so create instance and then access it.
So i have coded a method on eclipse (java), and I want to test if it works correctly, how do I do this, because the program won't run unless it has a main header.
So I guess what im asking is how do i use a method in another code
Well if your method is static you can access it via it's class name, if its a member of the class you have to create a instance of the class and call it using the instance.
Let's say we have this class and we want to call both methods in another class:
public class ClassToTest {
public static void staticMethodToTest(){
//Some code
}
public void memberMethodToTest(){
//Some code
}
}
To test them you can create another class:
public class MyClass {
//Create a main method so you can run your code
public static void main(String[] args) {
//Call static method
ClassToTest.staticMethodToTest();
//Call member
//Create instance of class
ClassToTest classToTestInstance = new ClassToTest();
//Call method on instance
classToTestInstance.memberMethodToTest();
}
}
In the case that both classes are in different packages you have to import the ClassToTest using import package.name.ClassToTest;
I think you want to test the internal workings of your program, you can do this by either creating a temporary main() method, or by using JUnit testing.
You can accomplish this simply by doing:
public static void main(String[] args)
{
//Test it here
}
and you should be able to just comment it out/remove it if it works.
//Have the method declared inside a class.
public MyClass(){
public String myString(String m){
return m;
}
}
//Create another class for your main method and inherit from MyClass
public MainClass extends MyClass {
public static void main(String[] args){
//Create an object obj from MyClass()
//and call the method on the object
MyClass obj = new MyClass();
System.out.println(obj.myString("hello"));
}
}
Write the method in another class, and give it a constructor so that it can be instantiated. In your main class, make an instance of the class and call your method from it.
I wrote a method just under my main method,
public static LinkList getContents()
then in the main method LinkList list = getContents()
It could only work when I add static in the declaration of getContents, why?
otherwise it will report an error !
A non-static method has to be called on a specific instance of the class e.g. anObject.getContents().
It's a very important concept in Java! Because when calling or referencing things inside of a static method (such as main), you can only reference other static variables, methods, and objects.
Conversely, you can still reference static data from inside non static methods.
The solution for this would be to make an "object" of your class. This starts to get into one of the core concepts of object oriented programming.
Making an object of a class (inside of main) :
ClassName ->pick any name<- = new ClassName();
then you can reference the method like this ->the name you chose.getContents();
Here's some practice code
public class Person{
public void setName(String name){
...
}
public static void main(String[] args){
Person bob = new Person();
bob.setName("pete");
}
}
If static methods are resolved at compile time how is an object instance able to call a static method?
class StaticCall
{
public static void main(String[] args)
{
String arr[]={"Class StaticCall","calls static method of class MyMainClass"};
MyMainClass h=new MyMainClass();
h.main(arr); //How is an instance able to call a static method?
System.out.println("this is StaticCall main");
}
}
class MyMainClass
{
public static void main(String[] args){
System.out.println(args[0]+" "+ args[1]);
}
}
After running the StaticCall class the output is
Class StaticCall calls static method of class MyMainClass
this is StaticCall main
As static fields and methods belong to the Class object how is an instance able to call a static method?
Also when is the Class object created,Is it on first access to any of it's fields or methods?
How is an instance able to call a static method?
It doesn't. Try this instead
MyMainClass h = null;
h.main(arr);
and you will see that the instance is ignored as this is exactly the same as
MyMainClass.main(arr);
To extend your example ... if you have
class AnotherMainClass extends MyMainClass
{
}
then all the following call the same method.
AnotherMainClass amc = null;
amc.main(args);
((AnotherMainClass) null).main(args);
AnotherMainClass.main(args);
MyMainClass mmc = null;
mmc.main(args);
((MyMainClass) null).main(args);
MyMainClass.main(args);
h.main(arr); //How is an instance able to call a static method?
This is just a shortcut for MyMainClass.main(arr), i.e. the static type of h. The usage is often frowned upon and most IDEs will recommend you use the type instead of instance.
Since this occurs at compile time, h can be null
you can call static method by classname.staticMethod and even instance.staticMethod, instance.staticMethod internally call classname.staticMethod.
This question already has answers here:
Cannot make a static reference to the non-static method
(8 answers)
Closed 5 years ago.
I'm getting an error when I try to call a non-static method in a static class.
Cannot make a static reference to the non-static method methodName() from the type playback
I can't make the method static as this gives me an error too.
This static method cannot hide the instance method from xInterface
Is there any way to get round calling an non-static method in another static method? (The two methods are in seperate packages and seperate classes).
The only way to call a non-static method from a static method is to have an instance of the class containing the non-static method. By definition, a non-static method is one that is called ON an instance of some class, whereas a static method belongs to the class itself.
You could create an instance of the class you want to call the method on, e.g.
new Foo().nonStaticMethod();
Firstly create a class Instance and call the non-static method using that instance.
e.g,
class demo {
public static void main(String args[]) {
demo d = new demo();
d.add(10,20); // to call the non-static method
}
public void add(int x ,int y) {
int a = x;
int b = y;
int c = a + b;
System.out.println("addition" + c);
}
}
public class StaticMethod{
public static void main(String []args)throws Exception{
methodOne();
}
public int methodOne(){
System.out.println("we are in first methodOne");
return 1;
}
}
the above code not executed because static method must have that class reference.
public class StaticMethod{
public static void main(String []args)throws Exception{
StaticMethod sm=new StaticMethod();
sm.methodOne();
}
public int methodOne(){
System.out.println("we are in first methodOne");
return 1;
}
}
This will be definitely get executed. Because here we are creating reference which nothing but "sm" by using that reference of that class which is nothing
but (StaticMethod=new Static method()) we are calling method one (sm.methodOne()).
I hope this will be helpful.
You need an instance of the class containing the non static method.
Is like when you try to invoke the non-static method startsWith of class String without an instance:
String.startsWith("Hello");
What you need is to have an instance and then invoke the non-static method:
String greeting = new String("Hello World");
greeting.startsWith("Hello"); // returns true
So you need to create and instance to invoke it.
It sounds like the method really should be static (i.e. it doesn't access any data members and it doesn't need an instance to be invoked on). Since you used the term "static class", I understand that the whole class is probably dedicated to utility-like methods that could be static.
However, Java doesn't allow the implementation of an interface-defined method to be static. So when you (naturally) try to make the method static, you get the "cannot-hide-the-instance-method" error. (The Java Language Specification mentions this in section 9.4: "Note that a method declared in an interface must not be declared static, or a compile-time error occurs, because static methods cannot be abstract.")
So as long as the method is present in xInterface, and your class implements xInterface, you won't be able to make the method static.
If you can't change the interface (or don't want to), there are several things you can do:
Make the class a singleton: make the constructor private, and have a static data member in the class to hold the only existing instance. This way you'll be invoking the method on an instance, but at least you won't be creating new instances each time you need to call the method.
Implement 2 methods in your class: an instance method (as defined in xInterface), and a static method. The instance method will consist of a single line that delegates to the static method.
The only way to call a non-static method from a static method is to have an instance of the class containing the non-static method.
class A
{
void method()
{
}
}
class Demo
{
static void method2()
{
A a=new A();
a.method();
}
/*
void method3()
{
A a=new A();
a.method();
}
*/
public static void main(String args[])
{
A a=new A();
/*an instance of the class is created to access non-static method from a static method */
a.method();
method2();
/*method3();it will show error non-static method can not be accessed from a static method*/
}
}
There are two ways:
Call the non-static method from an instance within the static method. See fabien's answer for an oneliner sample... although I would strongly recommend against it. With his example he creates an instance of the class and only uses it for one method, only to have it dispose of it later. I don't recommend it because it treats an instance like a static function.
Change the static method to a non-static.
You can't get around this restriction directly, no. But there may be some reasonable things you can do in your particular case.
For example, you could just "new up" an instance of your class in the static method, then call the non-static method.
But you might get even better suggestions if you post your class(es) -- or a slimmed-down version of them.
The easiest way to use a non-static method/field within a a static method or vice versa is...
(To work this there must be at least one instance of this class)
This type of situation is very common in android app development eg:- An Activity has at-least one instance.
public class ParentClass{
private static ParentClass mParentInstance = null;
ParentClass(){
mParentInstance = ParentClass.this;
}
void instanceMethod1(){
}
static void staticMethod1(){
mParentInstance.instanceMethod1();
}
public static class InnerClass{
void innerClassMethod1(){
mParentInstance.staticMethod1();
mParentInstance.instanceMethod1();
}
}
}
Note:- This cannot be used as a builder method like this one.....
String.valueOf(100);
I use an interface and create an anonymous instance of it like so:
AppEntryPoint.java
public interface AppEntryPoint
{
public void entryMethod();
}
Main.java
public class Main
{
public static AppEntryPoint entryPoint;
public static void main(String[] args)
{
entryPoint = new AppEntryPoint()
{
//You now have an environment to run your app from
#Override
public void entryMethod()
{
//Do something...
System.out.println("Hello World!");
}
}
entryPoint.entryMethod();
}
public static AppEntryPoint getApplicationEntryPoint()
{
return entryPoint;
}
}
Not as elegant as creating an instance of that class and calling its own method, but accomplishes the same thing, essentially. Just another way to do it.
It is not possible to call non-static method within static method. The logic behind it is we do not create an object to instantiate static method, but we must create an object to instantiate non-static method. So non-static method will not get object for its instantiation inside static method, thus making it incapable for being instantiated.
Constructor is a special method which in theory is the "only" non-static method called by any static method. else its not allowed.
You can call a non static method within a static one using:
Classname.class.method()