attempting to use incompatible return type

We may encounter this error message when we override a superclass method in the subclass with a wrong return type in the subclass method. To understand this, let's see the following two classes.

public class SuperClass
{
public double sum(double a,double b)
{
return (a+b);
}
}

public class Subclass extends SuperClass
{
public long sum(double a,double b)
{
return Math.round(a+b);
}
}

In the above example code, the super class method sum has double return type and in the subclass we have overridden this method where return type is long which is not compatible with double.
What should we do now to remove the error? We should either declare the return type of the subclass as double or change the parameter type.

See the correct code of the subclass below:

public class Subclass extends SuperClass
{
public double sum(double a,double b)
{
return Math.round(a+b);
}
public long sum(long a,long b)
{
return (a+b);
}
}

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is not abstract and does not override abstract method

See the code below:

import javax.swing.JFrame;
import javax.swing.JButton;
import java.awt.Container;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class MyFrame extends JFrame implements ActionListener{
private JButton exitButton = new JButton("Exit");
public MyFrame()
{
Container container = getContentPane();
container.setLayout(new FlowLayout());
container.add(exitButton);
  exitButton.addActionListener(this);
setSize(300,200);
}
}

If this code is compiled, we will see the error "MyFrame is not abstract and does not override abstract method actionPerformed(java.awt.event.ActionEvent) in java.awt.event.ActionListener"

Why? When we implement an interface(ActionListener in this example) or extend an abstract class in a concrete class, we must override the abstract method(s). In the above example we have implemented ActionListener but have not overridden the abstract method actionPerformed.

The correct code of the above is:

import javax.swing.JFrame;
import javax.swing.JButton;
import java.awt.Container;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class MyFrame extends JFrame implements ActionListener
{
private JButton exitButton = new JButton("Exit");
public MyFrame()
{
Container container = getContentPane();
container.setLayout(new FlowLayout());
container.add(exitButton);
exitButton.addActionListener(this);
setSize(300,200);
}
public void actionPerformed(ActionEvent ae)
{
System.exit(0);
}
}

Sometimes it happens that we override the abstract method with wrong name spelling or wrong signature, then also we see the same error message. Note that Java is a case sensitive language; most often we write the method name with wrong case (example, actionperformed/ActionPerformed instead of actionPerformed) and see this error message.

For implementing an interface or extending an abstract class, we must override all of the abstract methods (if more than one), though we do not need all. For example, if we implement KeyListener we must override each of the three abstract methods keyPressed, keyTyped and keyReleased though we may not need all of them. In that case, method body can be kept blank. Instead of interface, we can use adapter class (if available) to get rid of overriding all abstract methods.

In summary, override the abstract method with correct name spelling and signature to remove the error.

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attempting to assign weaker access privileges

Java error "attempting to assign weaker access privileges" can happen when we override a superclass method in the subclass if the access modifier is not correctly chosen.

See the sample code below:

public class SuperClass
{
public void display()
{
System.out.println("this is super class");
}
}

public class Subclass extends SuperClass
{
private void display()
{
System.out.println("this is subclass");
}
}

If the above Subclass is compiled the error message will be:
display() in Subclass cannot override display() in SuperClass; attempting to assign weaker access privileges; was public

Let's understand the error message:
The method display has public access in the SuperClass and private access in the Subclass, that means when overriding this method, a weaker access privilege is going to be set (private is weaker access privilege than public).

So we need to know the allowed access modifier in the subclass for method overriding. The table below shows this:

Access modifier in super class method	Allowed access modifier for overriding
private	private, package, protected, public
package	package, protected, public
protected	protected, public
public	public

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