Check out example codes for "calculator program in java". It will help you in understanding the concepts better.

Code Example 1

import java.util.Scanner;
public class Calculator {
   public static void main(String[] args) {
      double num1;
      double num2;
      double ans;
      char op;
      Scanner reader = new Scanner(System.in);
      System.out.print("Enter two numbers: ");
      num1 = reader.nextDouble();
      num2 = reader.nextDouble();
      System.out.print("\nEnter an operator (+, -, *, /): ");
      op = reader.next().charAt(0);
      switch(op) {
         case '+': ans = num1 + num2;
            break;
         case '-': ans = num1 - num2;
            break;
         case '*': ans = num1 * num2;
            break;
         case '/': ans = num1 / num2;
            break;
      default: System.out.printf("Error! Enter correct operator");
         return;
      }
      System.out.print("\nThe result is given as follows:\n");
      System.out.printf(num1 + " " + op + " " + num2 + " = " + ans);
   }
}

Code Example 2

Save this file as MyCalculator.java 
to compile it use 
    javac MyCalculator.java 
to use the calcuator do this 
    java MyCalculator 
 
**********************************************/  
import java.awt.*;  
import java.awt.event.*;  
/*********************************************/  
  
public class MyCalculator extends Frame  
{  
  
public boolean setClear=true;  
double number, memValue;  
char op;  
  
String digitButtonText[] = {"7", "8", "9", "4", "5", "6", "1", "2", "3", "0", "+/-", "." };  
String operatorButtonText[] = {"/", "sqrt", "*", "%", "-", "1/X", "+", "=" };  
String memoryButtonText[] = {"MC", "MR", "MS", "M+" };  
String specialButtonText[] = {"Backspc", "C", "CE" };  
  
MyDigitButton digitButton[]=new MyDigitButton[digitButtonText.length];  
MyOperatorButton operatorButton[]=new MyOperatorButton[operatorButtonText.length];  
MyMemoryButton memoryButton[]=new MyMemoryButton[memoryButtonText.length];  
MySpecialButton specialButton[]=new MySpecialButton[specialButtonText.length];  
  
Label displayLabel=new Label("0",Label.RIGHT);  
Label memLabel=new Label(" ",Label.RIGHT);  
  
final int FRAME_WIDTH=325,FRAME_HEIGHT=325;  
final int HEIGHT=30, WIDTH=30, H_SPACE=10,V_SPACE=10;  
final int TOPX=30, TOPY=50;  
///////////////////////////  
MyCalculator(String frameText)//constructor  
{  
super(frameText);  
  
int tempX=TOPX, y=TOPY;  
displayLabel.setBounds(tempX,y,240,HEIGHT);  
displayLabel.setBackground(Color.BLUE);  
displayLabel.setForeground(Color.WHITE);  
add(displayLabel);  
  
memLabel.setBounds(TOPX,  TOPY+HEIGHT+ V_SPACE,WIDTH, HEIGHT);  
add(memLabel);  
  
// set Co-ordinates for Memory Buttons  
tempX=TOPX;   
y=TOPY+2*(HEIGHT+V_SPACE);  
for(int i=0; i<memoryButton.length; i++)  
{  
memoryButton[i]=new MyMemoryButton(tempX,y,WIDTH,HEIGHT,memoryButtonText[i], this);  
memoryButton[i].setForeground(Color.RED);  
y+=HEIGHT+V_SPACE;  
}  
  
//set Co-ordinates for Special Buttons  
tempX=TOPX+1*(WIDTH+H_SPACE); y=TOPY+1*(HEIGHT+V_SPACE);  
for(int i=0;i<specialButton.length;i++)  
{  
specialButton[i]=new MySpecialButton(tempX,y,WIDTH*2,HEIGHT,specialButtonText[i], this);  
specialButton[i].setForeground(Color.RED);  
tempX=tempX+2*WIDTH+H_SPACE;  
}  
  
//set Co-ordinates for Digit Buttons  
int digitX=TOPX+WIDTH+H_SPACE;  
int digitY=TOPY+2*(HEIGHT+V_SPACE);  
tempX=digitX;  y=digitY;  
for(int i=0;i<digitButton.length;i++)  
{  
digitButton[i]=new MyDigitButton(tempX,y,WIDTH,HEIGHT,digitButtonText[i], this);  
digitButton[i].setForeground(Color.BLUE);  
tempX+=WIDTH+H_SPACE;  
if((i+1)%3==0){tempX=digitX; y+=HEIGHT+V_SPACE;}  
}  
  
//set Co-ordinates for Operator Buttons  
int opsX=digitX+2*(WIDTH+H_SPACE)+H_SPACE;  
int opsY=digitY;  
tempX=opsX;  y=opsY;  
for(int i=0;i<operatorButton.length;i++)  
{  
tempX+=WIDTH+H_SPACE;  
operatorButton[i]=new MyOperatorButton(tempX,y,WIDTH,HEIGHT,operatorButtonText[i], this);  
operatorButton[i].setForeground(Color.RED);  
if((i+1)%2==0){tempX=opsX; y+=HEIGHT+V_SPACE;}  
}  
  
addWindowListener(new WindowAdapter()  
{  
public void windowClosing(WindowEvent ev)  
{System.exit(0);}  
});  
  
setLayout(null);  
setSize(FRAME_WIDTH,FRAME_HEIGHT);  
setVisible(true);  
}  
//////////////////////////////////  
static String getFormattedText(double temp)  
{  
String resText=""+temp;  
if(resText.lastIndexOf(".0")>0)  
    resText=resText.substring(0,resText.length()-2);  
return resText;  

Code Example 3

// this was all coded by me, to use it get Eclipse, or IntelliJ and make a new
// Java Project

package javaCode;

import java.util.Scanner;

public class testCode {

	public static void main(String[] args) {
		
		Scanner scan = new Scanner(System.in);
		
		System.out.println("Would you like to Add, Subtract, Divide, or Multiply?");
		
		String mathType = scan.next();
		
		if (mathType.equalsIgnoreCase("add")) {
			System.out.println("Whats the first number you want to add?");
			
			int firstNumber = scan.nextInt();
			
			System.out.println(firstNumber + " + ");
			System.out.println("Enter your second number");
			
			int secondNumber = scan.nextInt();
			
			System.out.println(firstNumber + " + " + secondNumber + " =");
			System.out.println(firstNumber + secondNumber);
			
		} else if (mathType.equalsIgnoreCase("subtract")) {
			System.out.println("Whats the first number you want to subtract?");
			
			int firstNumber = scan.nextInt();
			
			System.out.println(firstNumber + " - ");
			System.out.println("Enter your second number");
			
			int secondNumber = scan.nextInt();
			
			System.out.println(firstNumber + " - " + secondNumber + " =");
			System.out.println(firstNumber - secondNumber);
		} else if (mathType.equalsIgnoreCase("divide")) {
			System.out.println("Whats the first number you want to divide?");
			
			int firstNumber = scan.nextInt();
			
			System.out.println(firstNumber + " % ");
			System.out.println("Enter your second number");
			
			int secondNumber = scan.nextInt();
			
			System.out.println(firstNumber + " % " + secondNumber + " =");
			System.out.println(firstNumber / secondNumber);
		} else if (mathType.equalsIgnoreCase("multiply")){
			System.out.println("Whats the first number you want to multiply?");
			
			int firstNumber = scan.nextInt();
			
			System.out.println(firstNumber + " x ");
			System.out.println("Enter your second number");
			
			int secondNumber = scan.nextInt();
			
			System.out.println(firstNumber + " x " + secondNumber + " =");
			System.out.println(firstNumber * secondNumber);
		}
		
	}

}

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