Java is an object-oriented programming language that supports encapsulation, inheritance, and polymorphism. In Java, a variable is a named storage location that holds a value. The language has several built-in data types, including byte, short, int, long, float, double, boolean, and char. Java also supports various operators for performing arithmetic, comparison, logical, and assignment operations.
Control structures determine the flow of a program's execution. Java supports conditional statements like if-else and switch, loops like for, while, and do-while, and jump statements like break, continue, and return. Functions in Java are called methods, which are blocks of code that perform specific tasks. A method declaration includes an access modifier, return type, method name, and parameters.
In Java, a class is a blueprint for creating objects. An object is an instance of a class, and it has attributes (data members) and methods (functions that belong to the class). Inheritance allows one class to inherit properties and behavior from another class. The subclass inherits all the fields and methods of the superclass and can also add new fields and methods or override the ones inherited from the superclass.
Polymorphism is another key feature of Java that allows objects of different classes to be treated as objects of a common superclass. This is achieved through method overriding, where a subclass provides a different implementation of a method, and method overloading, where multiple definitions of a method are provided. Java also supports exception handling, which allows a program to handle runtime errors and exceptions. The try-catch block is used to catch and handle exceptions, while the throw and throws keywords are used to throw and declare exceptions, respectively.
Let's take a simple Java program as an example:
# Hello World Java Program
```
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, World!");
}
}
```
Here's a breakdown of the code:
1. `public class HelloWorld`
- `public`: Access modifier, meaning the class can be accessed from anywhere.
- `class`: Keyword to define a new class.
- `HelloWorld`: Name of the class.
2. `public static void main(String[] args)`
- `public`: Access modifier, same as above.
- `static`: Keyword meaning the method can be called without creating an instance of the class.
- `void`: Return type, indicating the method doesn't return any value.
- `main`: Name of the method, which is the entry point of the program.
- `String[] args`: Parameter, an array of strings representing command-line arguments.
3. `System.out.println("Hello, World!");`
- `System.out`: Output stream, used to print text to the console.
- `println`: Method, short for "print line", which prints the text followed by a newline character.
- `"Hello, World!"`: String literal, the text to be printed.
How it works
1. The Java Virtual Machine (JVM) loads the `HelloWorld` class.
2. The JVM calls the `main` method, passing any command-line arguments.
3. The `main` method executes, printing "Hello, World!" to the console.
This is a very basic Java program, but it demonstrates the basic structure and syntax of a Java program.
Let's dive deeper into Java programming concepts.
# Variables and Data Types
In Java, a variable is a named storage location that holds a value. Java has several built-in data types:
- Byte (8-bit signed integer)
- Short (16-bit signed integer)
- Int (32-bit signed integer)
- Long (64-bit signed integer)
- Float (32-bit floating-point number)
- Double (64-bit floating-point number)
- Boolean (true or false)
- Char (16-bit Unicode character)
# Operators
Java supports various operators for performing arithmetic, comparison, logical, and assignment operations:
- Arithmetic operators: +, -, *, /, %
- Comparison operators: ==, !=, <, >, <=, >=
- Logical operators: &&, ||, !
- Assignment operators: =, +=, -=, *=, /=, %=
# Control Structures
Control structures determine the flow of a program's execution:
- Conditional statements: if-else, switch
- Loops: for, while, do-while
- Jump statements: break, continue, return
# Functions
In Java, functions are called methods. They are blocks of code that perform specific tasks:
- Method declaration: access modifier, return type, method name, parameters
- Method call: method name followed by parentheses containing arguments
# Classes and Objects
Java is an object-oriented programming language that supports encapsulation, inheritance, and polymorphism:
- Class: a blueprint for creating objects
- Object: an instance of a class
- Attributes: data members of a class
- Methods: functions that belong to a class
# Inheritance
Inheritance allows one class to inherit properties and behavior from another class:
- Superclass: the parent class
- Subclass: the child class
- Extends keyword: used to inherit from a superclass
# Polymorphism
Polymorphism allows objects of different classes to be treated as objects of a common superclass:
- Method overriding: providing a different implementation of a method
- Method overloading: providing multiple definitions of a method
# Exception Handling
Exception handling allows a program to handle runtime errors and exceptions:
- Try-catch block: used to catch and handle exceptions
- Throw keyword: used to throw an exception
- Throws keyword: used to declare exceptions.
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