Java Cheat Sheet

A quick reference of the most used Java syntax. Use it to revise before an exam or an interview. Every example is tested on Java 25.

Program

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

Variables and types

Type	Example
int, long	int n = 5; long big = 5_000_000_000L;
double, float	double d = 3.14; float f = 3.14f;
char, boolean	char c = 'A'; boolean ok = true;
String	String s = "text";
var	var list = new ArrayList<String>();
Constant	final int MAX = 10;
Cast	int x = (int) 3.9;

Operators

Group	Operators
Arithmetic	+ - * / % ++ --
Comparison	== != > < >= <=
Logical	&& || !
Assignment	= += -= *= /= %=
Ternary	cond ? a : b

Control flow

public class Flow {
    public static void main(String[] args) {
        int n = 7;
        if (n > 5) System.out.println("big"); else System.out.println("small");

        String name = switch (n % 3) {
            case 0 -> "zero";
            case 1 -> "one";
            default -> "two";
        };
        System.out.println(name);

        for (int i = 0; i < 3; i++) System.out.print(i + " ");
        for (String s : new String[]{"a", "b"}) System.out.print(s + " ");
        int k = 0;
        while (k < 2) { System.out.print("w" + k + " "); k++; }
        System.out.println();
    }
}

Strings

Code	Result
s.length()	Number of characters
s.charAt(0)	First character
s.substring(1, 3)	Part of the text
s.indexOf("x")	Position, or -1
s.equals(t)	Compare the text
s.split(",")	Array of parts
String.format("%.2f", d)	Formatted text

Arrays and collections

import java.util.*;

public class Coll {
    public static void main(String[] args) {
        int[] arr = {3, 1, 2};
        Arrays.sort(arr);
        System.out.println(Arrays.toString(arr));

        List<String> list = new ArrayList<>(List.of("b", "a"));
        Collections.sort(list);
        System.out.println(list);

        Map<String, Integer> map = new HashMap<>();
        map.put("x", 1);
        map.merge("x", 5, Integer::sum);
        System.out.println(map);

        Set<Integer> set = new TreeSet<>(List.of(3, 1, 3));
        System.out.println(set);
    }
}

Classes

interface Speaker { String speak(); }

abstract class Animal implements Speaker {
    protected final String name;
    Animal(String name) { this.name = name; }
}

class Dog extends Animal {
    Dog(String name) { super(name); }
    @Override public String speak() { return name + " says Woof"; }
}

record Point(int x, int y) { }

enum Level { LOW, HIGH }

public class Cls {
    public static void main(String[] args) {
        System.out.println(new Dog("Tommy").speak());
        System.out.println(new Point(1, 2));
        System.out.println(Level.HIGH);
    }
}

Exceptions

public class Exc {
    public static void main(String[] args) {
        try {
            Integer.parseInt("x");
        } catch (NumberFormatException e) {
            System.out.println("bad number");
        } finally {
            System.out.println("always");
        }
    }
}

Lambdas and streams

import java.util.List;

public class Lam {
    public static void main(String[] args) {
        List<Integer> n = List.of(1, 2, 3, 4, 5, 6);
        System.out.println(n.stream().filter(x -> x % 2 == 0).map(x -> x * x).toList());
        System.out.println(n.stream().mapToInt(Integer::intValue).sum());
    }
}

Commands

javac Main.java        # compile
java Main              # run the compiled class
java Main.java         # compile and run a single file
java -version          # Java version

Where to go next

• Practice with the Java exercises and the interview questions of this website.
• Learn a build tool (Maven or Gradle) and a framework like Spring Boot.
• Build small projects: a to do list, a bank account, a library system.

• Previous Page

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Exercises & Assignments
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Interview Questions & Answers
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