Introduction to Programming in Java

Every app we use is built with programming

Each one is a set of instructions written by programmers in a programming language. Programming is writing ordered instructions that a computer follows to solve a problem or perform a task. Today we write our first instructions in Java.

A calculator takes input, processes it and shows a result

Input, then processing, then output: a pattern you will see in most programs you write.

public class Calculator {
    public static void main(String[] args) {
        System.out.println(2 + 3);
    }
}

5

A computer follows instructions literally; it does not guess

Clear, ordered instructions are a core part of programming. Predict the output of the next two programs before revealing the answer.

public class Literal {
    public static void main(String[] args) {
        System.out.println("Helo, Jaba!");
    }
}

Hello, Java!

Helo, Jaba!

Error

Helo, Jaba!

public class Order {
    public static void main(String[] args) {
        System.out.println("C");
        System.out.println("A");
        System.out.println("B");
    }
}

A
B
C

C
A
B

C A B

C
A
B

Learning Java is like learning a new language

Java introduces ideas shared by many languages, such as variables, conditions and loops. Each language still has its own rules; knowing Java does not mean you know every language.

A variable is a name that holds a value the program uses

• A program often needs to store values while it runs.
• The number 2 is not a variable; the variable is the name that holds it.
• The value can change later, but the name stays the same.
• Variables are not limited to numbers.

public class TwoNumbers {
    public static void main(String[] args) {
        int firstNumber = 2;
        int secondNumber = 3;
        System.out.println(firstNumber + secondNumber);
    }
}

5

Three parts work together whenever a program runs

Three terms you need before we start

Where do we write a program, and how do we run it in NetBeans 8? We start by separating three names you will hear often.

Java 8

JDK 8u111

NetBeans IDE 8.2

A program passes through four stops: .java, javac, .class, then the JVM

1. Source file: FirstProgram.javaThis is where we write instructions in Java. It is text that people read.
2. The compiler: javacIt checks the code against the rules. If it finds an error it stops and shows a message; otherwise it translates the code.
3. Bytecode: FirstProgram.classThe compiler produces a file of bytecode: instructions that the Java Virtual Machine understands.
4. The Java Virtual Machine: JVMThe JVM runs the bytecode on your device, using the operating system and the CPU.

public class Path {
    public static void main(String[] args) {
        System.out.println("Compiled, then run by the JVM");
    }
}

Compiled, then run by the JVM

Create your first NetBeans 8 project in five steps, then use only three areas

1. Open the New Project windowFile, then New Project, or Ctrl+Shift+N.
2. Choose the project typeJava, then Java Application, then Next.
3. Name the projectFirstProgram. Keep Create Main Class checked, then Finish.
4. Open the main classFirstProgram.java opens automatically, with main in it.
5. Run the projectRun Project, or F6, or the green Run button.

Output - FirstProgram (run)
run:
Hello, Java!
BUILD SUCCESSFUL (total time: 0 seconds)

Your first program: one line that prints a message

1. Write the line inside the braces of main
2. Predict the output before running
3. Run itIn the lab press F6; here use the Run button or Ctrl+Enter.
4. Change the message and run again

package firstprogram;

public class FirstProgram {
    public static void main(String[] args) {
        System.out.println("Hello, Java!");
    }
}

Hello, Java!

public class Welcome {
    public static void main(String[] args) {
        System.out.println("Hello, Java!");
    }
}

Welcome to Java

public class Welcome {
    public static void main(String[] args) {
        System.out.println("Welcome to Java");
    }
}

The "Debug line by line" button shows the program running each line

This site has a debugger on every program: it pauses before each line so you can watch the program work step by step. It is the best way to understand a program that does not do what you expected.

1. Press "Debug line by line"The program pauses and highlights the line about to run. It has not run yet.
2. Press "Next line" or F8The line runs. Look at the output below the editor, then predict what comes next.
3. Press "Continue" or F9The program runs the remaining lines without stopping.

public class Trace {
    public static void main(String[] args) {
        System.out.println("Line one");
        System.out.println("Line two");
        System.out.println(2 + 3);
    }
}

Line one
Line two
5

Quotation marks change the meaning of what we write

public class NoQuotes {
    public static void main(String[] args) {
        System.out.println(2 + 3);
    }
}

5

2 + 3

23

5

public class WithQuotes {
    public static void main(String[] args) {
        System.out.println("2 + 3");
    }
}

5

2 + 3

"2 + 3"

2 + 3

public class Quotes {
    public static void main(String[] args) {
        System.out.println(2 + 3);
        System.out.println("2 + 3");
    }
}

5
2 + 3

public class SumTwice {
    public static void main(String[] args) {
        // Write two println statements here

    }
}

4 + 6
10

public class SumTwice {
    public static void main(String[] args) {
        System.out.println("4 + 6");
        System.out.println(4 + 6);
    }
}

An error message shows us where to fix the code

An error is information that helps us fix the code, and seeing errors while learning is normal. In NetBeans a red mark appears next to the line number, with a red underline under the error. Hover over the mark to read the message.

public class Welcome {
    public static void main(String[] args) {
        System.out.println("Welcome")
    }
}

public class Welcome {
    public static void main(String[] args) {
        System.out.println("Welcome");
    }
}

Welcome

public class Welcome {
    public static void main(String[] args) {
        System.out.println("Welcome);
    }
}

public class Welcome {
    public static void main(String[] args) {
        System.out.println("Welcome");
    }
}

Welcome

A routine for every exercise, plus time-saving shortcuts

public class Routine {
    public static void main(String[] args) {
        System.out.println("10 + 5");
        System.out.println(10 + 5);
        System.out.println("Done");
    }
}

10 + 5
15
Done

• New to the keyboard? Switch the keyboard to English before typing code (Alt+Shift).
• Parentheses ( ): Shift + 9 and 0.
• Braces { }: Shift + the two keys right of P.
• Quotation mark ": Shift + the key right of the semicolon key.
• Semicolon ;: the key right of L.

Final exercise: a program that prints three lines

public class FirstProgram {
    public static void main(String[] args) {
        // Write your three println statements here

    }
}

Ahmed
My first Java program
15

public class FirstProgram {
    public static void main(String[] args) {
        System.out.println("Ahmed");
        System.out.println("My first Java program");
        System.out.println(10 + 5);
    }
}

Review questions

public class Review {
    public static void main(String[] args) {
        System.out.println("10 + 5");
        System.out.println(10 + 5);
    }
}

10 + 5
15

15
15

10 + 5
10 + 5

15
10 + 5

10 + 5
15