Compostion

[ehsan]

Hi Every one . I created two class Human and Employee and then create one object from human in side employee , but when run my program compiler return error
Exception in thread "main" java.lang.NullPointerException
at Employee.main(Employee.java:15)

public class Human{
	private String name;
	public void setName(String n){
	 name = n;
	}
	public String getName(){
		return name;
	}
}
public class Employee{
	private String id;
	public Human h;
	private void setEmployee(String id){
		this.id = id;
	}
	private String getEmployee() {
		return id;
	}
	
	public static void main(String[] args){
		Employee emp = new Employee();
        	emp.h.setName("ehsan");
	}
}

[sehudson]

you never initialized your human. You declared it with "public Human h;" But you have to initialize it before you can make reference to it, and the constructor is a good place to do that.

public class Employee{
private String id;
public Human h;

public Employee(){
h = new Human();
}

private void setEmployee(String id){
this.id = id;
}
private String getEmployee() {
return id;
}

public static void main(String[] args){
Employee emp = new Employee();
emp.h.setName("eshan");
}
}

[pbrockway2]

Please use code tags: put [code] at the start of the code and [/code] at the end. That way the code will be readable when it appears here.

Exception in thread "main" java.lang.NullPointerException
at Employee.main(Employee.java:15)

This exception means that you are trying to use something as if it had a value when it is really null. Common cases look like

foo[i];  // exception when foo is null
bar.baz(); // exception when bar is null

I am guessing line 15 is

emp.h.setName("eshan");

Check, by using System.out.println(), what the values of emp and h are. One of them is null! Once you have found the culprit (or by using sehudson's advice, but its good to check) search for whereever you thought you had assigned it a non null value and see why that didn't happen.

[ehsan]

Tnx.... it's working Nowwww