Program shape
Java programs run from a main method. Classes group related state and behavior.
class Main {
public static void main(String[] args) {
System.out.println("Hello, Java!");
}
}What you will learn
- Write and run Java programs using types, decisions, loops, and methods.
- Model real data with classes, constructors, encapsulation, and inheritance.
- Use arrays, strings, collections, files, generics, and exceptions.
- Apply streams, lambdas, tests, and debugging habits in practical code.
Practice method: open each sample in Open Editor, run it, then change one value and predict the result before running again.
1. Core language basics
Types and control flow
Use explicit types and braces around blocks. A for loop is ideal when the iteration range is known.
for (int number = 1; number <= 5; number++) {
if (number % 2 == 0) {
System.out.println(number);
}
}2. Objects and classes
Classes model meaningful state
Keep fields private and use methods to maintain valid object state.
class Counter {
private int value;
void increment() {
value++;
}
int value() {
return value;
}
}Inheritance should be intentional
Use inheritance for a genuine is-a relationship. Interfaces are useful for a shared capability.
interface Printable {
void print();
}3. Collections, files, and reliability
Use the Java Collections Framework for groups of data, try/catch for expected failures, and try-with-resources to close files automatically.
List<String> names = List.of("Ada", "Lin");
for (String name : names) {
System.out.println(name);
}Reliable Java: validate external input, catch specific exceptions, close resources automatically, and write tests for behavior that must not change.
Before you move on
- I can write a Java class with main.
- I can create and call methods.
- I can use constructors and private fields.
- I can select a collection for a data problem.
- I can handle a known exception.
- I can read a compiler error and fix it.