Java Examples: Programs by Task, Add Two Integers and More

Java examples are small, complete programs that solve one task each, such as adding two numbers, checking a prime number or reversing a string, so we can run them, read the output and change them while learning the language. This page groups all our example programs by task and starts with the first program most beginners write, adding two integers.

We use these programs to practice syntax and the standard library, to prepare for coding interviews, and as starting points for real code. The following example adds two integers and shows the one detail that beginner programs often miss, which is overflow.

int apples = 10;
int pears = 20;
int total = apples + pears;                       // 30
int wrapped = Integer.MAX_VALUE + 1;              // -2147483648, int overflow
long widened = (long) Integer.MAX_VALUE + 1;      // 2147483648
int checked = Math.addExact(apples, pears);       // 30, throws ArithmeticException on overflow

Notice that the plain + operator wraps around without any error when the result does not fit into an int. After the addition program and its input version, the collection lists the console and command-line examples, number programs, string programs and programs about JVM behavior, each with what it teaches.

1. Java Program to Add Two Integers

Adding two numbers teaches variables, the int type, operators and printing. An int holds values from -2,147,483,648 to 2,147,483,647, and the + operator on two int values gives an int result.

int first = 15;
int second = 27;
int sum = first + second;                        // 42
String message = "Sum: " + sum;                  // "Sum: 42"
String wrong = "Sum: " + first + second;         // "Sum: 1527", string concatenation

The last line is a frequent beginner mistake. Java evaluates + from left to right, so once the left side is a String, each further + appends text. Parentheses fix it, as in “Sum: ” + (first + second).

1.1. Adding Two Numbers Entered by the User

A program that reads the numbers from the keyboard wraps System.in in a Scanner. The safe version checks each token with hasNextInt() before reading it, so input such as ten leads to a message instead of an InputMismatchException. To keep the snippet runnable, the scanner reads from a string with the typed text.

Scanner input = new Scanner("15 abc 27");          // new Scanner(System.in) in a real program
int firstNumber = input.hasNextInt() ? input.nextInt() : 0;   // 15
boolean secondValid = input.hasNextInt();                     // false, "abc" is not a number
String rejected = input.next();                               // "abc"
int secondNumber = input.hasNextInt() ? input.nextInt() : 0;  // 27
int userSum = firstNumber + secondNumber;                     // 42

In a full program, the rejected token leads to a prompt such as Please enter a whole number and a new read inside a loop. The guide to reading input from the console shows that loop and the other ways to read user input. Since Java 25, a beginner program also fits in a few lines without a class declaration, as the Java hello world guide explains.

void main() {
    try {
        int a = Integer.parseInt(IO.readln("First number: ").strip());
        int b = Integer.parseInt(IO.readln("Second number: ").strip());
        IO.println("Sum: " + Math.addExact(a, b));
    } catch (NumberFormatException | ArithmeticException e) {
        IO.println("Please enter two whole numbers in the int range");
    }
}

1.2. Integer Overflow When Adding

When the true sum is larger than Integer.MAX_VALUE, the + operator keeps only the lowest 32 bits and the result wraps around to a negative number, without any error. Say a download manager adds the sizes of two files in bytes. Two files of 1.5 GB each add up to 3,221,225,472 bytes, which an int cannot hold.

int videoBytes = 1_610_612_736;                              // 1.5 GB
int intTotal = videoBytes + videoBytes;                      // -1073741824, wrapped around
long longTotal = (long) videoBytes + videoBytes;             // 3221225472
int exactTotal = Math.addExact(videoBytes, videoBytes);      // ArithmeticException: integer overflow

We use long for values that can grow past two billion, such as byte counts and timestamps in milliseconds, and Math.addExact() when an overflow must stop the program instead of producing a wrong number. The article on exact arithmetic methods covers subtractExact(), multiplyExact() and the rest, and BigInteger handles numbers of any size.

2. Console and Command-Line Programs

The console and command-line examples deal with what happens around the code, such as reading and writing text in a terminal, starting a program with its libraries and fixing the first error message beginners see.

Task	Example	What it teaches
Read and write text in the terminal	Console input and output	System.out, System.err, the Console class and password input
Start a program with its libraries	Set the classpath from the command line	-cp, CLASSPATH, wildcards and quoting per OS
Fix a failed start	Could not find or load main class	Package names, the classpath and the launcher

3. Number Programs

Number programs practice loops, the remainder operator % and integer division, which coding interviews often test early. Many of them check a special property of a number, and each article compares a plain loop with a stream-based version where that helps.

Task	Example	What it teaches
Average of N numbers	Calculate the average of N numbers	Integer vs floating-point division, streams
Swap two variables	Swap two numbers	Temporary variable, arithmetic and XOR swaps
Primes up to N	Prime numbers from 2 to N	Trial division, the sieve of Eratosthenes
Factorial	Calculate a factorial	Loops, recursion, BigInteger
FizzBuzz	FizzBuzz solution	Conditions and the % operator
Missing number in a series	Find the missing number	Sum formula and XOR
Disarium number	Disarium number	Digits and powers by position
Happy number	Happy and unhappy numbers	Cycle detection with a Set
Harshad number	Harshad number	Digit sums and divisibility
Pronic number	Pronic number	Products of consecutive integers
Deficient number	Deficient number	Divisor sums

4. String Programs

String programs train the methods of String, StringBuilder and Character, and they show how often a short stream pipeline replaces a loop. They are also common warm-up questions in Java interviews.

Task	Example	What it teaches
Sort strings	Sort a string array alphabetically	Arrays.sort(), comparators, case order
Reverse a string	Reverse a string	StringBuilder.reverse() and loops
Reverse with recursion	Reverse a string using recursion	Base case and recursive call
Reverse the words	Reverse words in a string	Splitting and joining
Count vowels and consonants	Count vowels and consonants	Character checks, streams
Palindrome string	Check a palindrome string	Two pointers
Palindrome number and string	Palindrome with a stack, a queue and loops	Several solutions compared
Remove extra spaces	Remove extra whitespaces	Regular expressions, strip()
Duplicate words	Find duplicate words	Map counts, grouping
Duplicate characters	Find duplicate characters	Frequency maps
Pangram check	Check if a string contains all letters	Sets and character ranges
Good and bad strings	Good string, bad string puzzle	Character rules in a loop

5. Programs About Java and JVM Behavior

The last group explains results that surprise developers with a few years of experience, such as two equal numbers that are not ==, or code that the compiler refuses as unreachable. Each one is short to run and teaches a rule of the language or the JVM.

Task	Example	What it teaches
Compare decimal numbers	Compare float and double correctly	Tolerances, Double.compare(), BigDecimal
127 == 127 but not 128	Wrapper class cache	The Integer cache and equals()
Low-level memory access	sun.misc.Unsafe and its replacements	VarHandle and the FFM API
Unreachable code	Dead code and unreachable code	Compiler flow analysis
Objects without new	Create an object without new	Reflection, cloning, deserialization
Loop in a linked list	Detect a loop in a linked list	Fast and slow pointers
A small game	Hi-Lo guessing game	Random numbers and a game loop

6. Java Examples FAQs

Beginners who work through these programs ask how to run them, where to start and how to vary the classic tasks.

6.1. How do we add two numbers in Java without the + operator?

We use bitwise operators. The XOR operator adds the bits without the carry, and AND shifted left by one gives the carry, so we repeat both steps until the carry is zero.

static int addWithoutPlus(int a, int b) {
    while (b != 0) {
        int carry = (a & b) << 1;
        a = a ^ b;
        b = carry;
    }
    return a;
}

int bitSum = addWithoutPlus(15, 27);              // 42

6.2. How do we run one of these examples?

We save the code in a file such as AddNumbers.java and start it with java AddNumbers.java. Since Java 11, the launcher compiles a single source file in memory, so no separate javac step is needed for small programs.

6.3. Which examples should a beginner start with?

We start with adding two integers and reading console input, continue with number programs such as the average, swapping and primes, and move on to the string programs after that. The Java tutorial covers the language basics that these programs use.

6.4. Should the sum of two int values be stored in a long?

Only when the sum can exceed about 2.1 billion. Casting one operand first, as in (long) a + b, makes the addition itself happen in long, whereas long sum = a + b; adds two int values and overflows before the assignment.

7. Conclusion

Small programs are the fastest way to learn how Java behaves, and the addition example already shows two lessons, string concatenation with + and silent int overflow, with long and Math.addExact() as the fixes.

The groups on this page move from the console and the command line to number and string programs and to JVM behavior.

8. References

• Learn Java, dev.java
• Math.addExact() Javadoc
• Scanner Javadoc
• JLS 25, Integer Operations
• JEP 512, Compact Source Files and Instance Main Methods

Happy Learning !!

Source Code on Github