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Java

Description

  • General-purpose programming language;

  • Object-oriented;

  • Runs on a virtual machine (JVM), so it's cross-platform;

  • Compiled to bytecode, which is then executed by the JVM.

History

  • Created by Sun Microsystems in 1995;

  • Open source since 2006;

  • Acquired by Oracle in 2010.

Principles

  1. It must be simple, object-oriented, and familiar;

  2. It must be robust and secure;

  3. It must be architecture-neutral and portable;

  4. It must execute with high performance;

  5. It must be interpreted, threaded, and dynamic.

Usage

  • Android apps;

  • Desktop apps;

  • Client-server web apps.

Inheritance

  • Inherit from a class using the extends keyword;

  • final classes cannot be extended;

  • Every class in Java inherits from Object;

  • Multiple inheritance is not supported.

Constructors in inheritance

  • If parent hasn't any constructor, child can define its own or not;

  • If no constructor is defined in both, the default constructor of each class is called;

  • If parent has one or more constructors, child must define at least one constructor, and it must call one of the parent's constructors using the super keyword.

Overriding methods

  • Use the @Override annotation to override a method.

public class Child extends Parent {
  @Override
  public void method() {
    // method() is already defined in Parent. We're overriding it.
  }
}

Error & Exception handling

Although it's safer to handle errors and exceptions, it slows down the program. So, it's better to handle only the exceptions that are likely to occur.

Checked & unchecked exceptions

  • Checked exceptions are exceptions that must be handled by the programmer;

  • Unchecked exceptions are exceptions that don't need to be handled by the programmer.

Error

  • Exceptional conditions that are external to the application, and that the application usually cannot anticipate or recover from;

  • Examples: JVM running out of memory, stack overflow, etc.

  • Errors are unchecked.

  • Errors are not meant to be caught or handled (although it's possible).

Exception

  • Exceptional conditions that are internal to the application, and that the application should anticipate and recover from;

  • Examples: invalid user input, file not found, network connection lost, etc.

  • Exceptions are checked.

RuntimeException

  • Exceptions that can be thrown anywhere in the code;

  • Examples: NullPointerException, IndexOutOfBoundsException, etc.

  • RuntimeExceptions are unchecked.

  • RuntimeExceptions are not meant to be caught or handled (although it's possible).

Errors & Exceptions hierarchy

Errors & Exceptions hierarchy

try-catch-finally

try {
  // Code that may throw an exception
} catch (Exception e) {
  // Code that handles the exception
} finally {
  // Code that always runs
}

Collections

List

  • Ordered collection of elements;

  • Implemented by:

    • ArrayList: Resizable array;

    • LinkedList: Doubly-linked list;

    • Vector: Thread-safe resizable array;

      • Stack: LIFO (last-in-first-out) collection.

Set

  • Unordered collection of unique elements;

  • Implemented by:

    • EnumSet: Abstract class for sets of enum types;

    • HashSet: Hash table;

    • SortedSet: Interface for sorted sets;

      • TreeSet: Red-black tree.

References

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