Live Code Classroom

INSTRUCTOR

⚊ OFFLINE

-- waiting for instructor --

INSTRUCTOR OUTPUT

📄 PROBLEM ⌄

OUTPUT

JAVA REFERENCE

Named containers that store a value. The type (int, double, boolean, String) tells Java what kind of data to expect.

int x = 5;
double d = 3.14;
boolean flag = true;
String s = "hello";

Prints text or values to the console. Use println to add a newline after, print to stay on the same line.

System.out.println("text");
System.out.print("no newline");
System.out.println("val: " + x);

Runs different code based on a condition. Use else if to check additional conditions, else to catch everything that didn't match.

if (x > 0) {
    // ...
} else if (x == 0) {
    // ...
} else {
    // ...
}

Repeat a block of code. Use for when you know the count, while when you have a condition, for-each to iterate a collection.

// for
for (int i = 0; i < 10; i++) { }

// while
while (condition) { }

// for-each
for (String item : list) { }

A fixed-size, ordered list of values all of the same type. Good when you know the size upfront and need fast index access.

int[] nums = {1, 2, 3};
nums[0]       // access
nums.length   // size

A resizable list from the Java standard library. Use it when the number of items can grow or shrink at runtime.

import java.util.ArrayList;
ArrayList<String> list = new ArrayList<>();
list.add("item");
list.get(0);
list.size();

Built-in actions you can call on any String to inspect or transform its text. Always use equals() to compare strings, not ==.

s.length()
s.toUpperCase()
s.substring(0, 3)
s.indexOf(",")
s.equals("other")

A class is a blueprint for objects. The constructor runs when you create a new instance and sets up its initial state.

public class Dog {
    private String name;

    public Dog(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }
}

Lets a child class reuse and extend a parent class. Use super() to call the parent constructor, @Override to replace a parent method.

public class Dog extends Animal {
    public Dog(String name) {
        super(name);  // call parent
    }
    @Override
    public String sound() {
        return "Woof!";
    }
}

A contract that any implementing class must fulfill. Use interfaces to define shared behavior across unrelated classes.

interface Printable {
    void print();
}

class Doc implements Printable {
    public void print() { ... }
}

A partial blueprint. Abstract methods have no body and must be implemented by subclasses. Use when subclasses share some code but differ on specifics.

abstract class Shape {
    public abstract double area();
}

class Circle extends Shape {
    public double area() {
        return Math.PI * r * r;
    }
}

Reads input typed by the user at the console. Always close the Scanner when done to release the underlying stream.

import java.util.Scanner;
Scanner sc = new Scanner(System.in);
String line = sc.nextLine();
int n = sc.nextInt();
sc.close();

Handles errors that might occur at runtime without crashing the program. The catch block runs only if the try block throws an exception.

try {
    // risky code
} catch (Exception e) {
    System.out.println(e.getMessage());
}