Applet compiles but doesn't work as intended

Question

My program intends to add, subtract, multiply or divide two numbers, whereby, if the user enters 'A' the program adds the two numbers, and 'S' to subtract them etc. The program compiles fine but as I am pretty new to Java I'm finding it hard to find my error. Reagrds in advance.
import java.applet.Applet;
import java.awt.*;
import java.awt.event.*;
import java.lang.*;
public class CharacterCalculation extends Applet implements ActionListener
     Label input1, input2, input3, output;
     TextField ip1, ip2, ip3, op;
     Double num1, number1, num2, number2, calculationa, calculations, calculationm, calculationd;
     String characterentered;
     public void init()
          setSize(280,180);
          input1 = new Label ("Enter first number here:");
          ip1 = new TextField(5);
          add(input1);
          add(ip1);
          ip1.addActionListener(this);
          number1=0.0;
          input2 = new Label ("Enter mathematical operator here:");
          ip2 = new TextField(5);
          add(input2);
          add(ip2);
          input3 = new Label ("Enter second number here:");
          ip3 = new TextField(5);
          add(input3);
          add(ip3);
          ip3.addActionListener(this);
          number2=0.0;
          output = new Label("The answer is:");
          op = new TextField(32);
          add(output);
          add(op);
          op.setEditable(false);
     public void paint (Graphics graf)
          graf.drawString("A = Add S = Subtract D = Divide M = Multiply",20,160);
     public void actionPerformed(ActionEvent ev)
          num1=Double.parseDouble(ip1.getText());
          num2=Double.parseDouble(ip3.getText());
          characterentered=(ip2.getText());
          calculationa=(num1+num2);
          calculations=(num1-num2);
          calculationm=(num1*num2);
          calculationd=(num1/num2);
          if (characterentered == "A")
               op.setText(Double.toString(calculationa));
          if (characterentered == "S")
               op.setText(Double.toString(calculations));
          if (characterentered == "M")
               op.setText(Double.toString(calculationm));
          if (characterentered == "D")
               op.setText(Double.toString(calculationd));
          else
               op.setText("Please enter a valid mathematical operator");
}

Answer 1

if (characterentered == "A")This is never going to work. A String is an object. What the above line does is compare the reference of characterentered to the reference of the object "A", which will never be the same (they are two different objects stored in two different memory locations).
If you want to compare two strings you need to use the equals() method, so change to four comparison lines to:
if(characterentered.equals("A")){
}