Java Crash Course

About Java

Java is a widely used computer programming language. It features cross-platform compatibility, object-oriented programming, and generic programming, and is widely used in enterprise-level Web application development and mobile application development.

Installation

See JDK.

Basic syntax

The main function

Like C/C++, Java needs a function (called a method in object-oriented programming) as the entry point for program execution.

The format of Java's main function is fixed, as follows:

class Test {
    public static void main(String[] args) {
        // Program code
    }
}

A packaged Java program (usually named *.jar) may contain many similar functions, but when the program is run, only one of them is executed. This is defined in the Manifest file of the Jar; this knowledge is generally not needed in OI competitions.

Comments

Like C/C++, Java uses // and /* */ for single-line and multi-line comments, respectively.

Basic data types

Declaring variables

int a = 12; // Set a to integer type and assign 12 to a
String str = "Hello, OI-wiki"; // Declare the string variable str
char ch = 'W';
double PI = 3.1415926;

The final keyword

final means that this is a final, unchangeable result. A variable modified by final can be assigned only once and cannot be changed after assignment.

final double PI = 3.1415926;

Arrays

// An integer array with ten elements
// Its syntax is: data type[] variable name = new data type[array size]
int[] ary = new int[10];

Strings

• A string is a built-in class in Java.

// The simplest way to construct a string variable is as follows
String a = "Hello";

// A string variable can also be constructed from a character array
char[] stringArray = { 'H', 'e', 'l', 'l', 'o' };
String s = new String(stringArray);

Packages and imports

In Java, classes (Class) are placed in packages (package). A package cannot contain two classes with the same name. The first line of a class usually specifies which package it belongs to. For example:

package org.oi-wiki.tutorial;

The usual package naming convention is: 项目所有者的顶级域.项目所有者的二级域.项目名称.

The import keyword is used to import classes that are not in the package to which the current class belongs. For example, the Scanner used below:

import java.util.Scanner;

To import all classes under a package, simply replace the class name before the final semicolon in this statement with *.

Input

The Scanner class can be used to process command-line input.

package org.oiwiki.tutorial;

import java.util.Scanner;

class Test {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in); // System.in is the input stream
        int a = scan.nextInt();
        double b = scan.nextDouble();
        String c = scan.nextLine();
    }
}

Output

Variables can be printed in formatted form.

class Test {
    public static void main(String[] args) {
        int a = 12;
        char b = 'A';
        double s = 3.14;
        String str = "Hello world";
        System.out.printf("%f\n", s);
        System.out.printf("%d\n", a);
        System.out.printf("%c\n", b);
        System.out.printf("%s\n", str);
    }
}

Control statements

Java's flow-control statements are basically the same as C++'s.

Selection

• if

class Test {
    public static void main(String[] args) {
        if ( /* condition */ ){
            // Code executed when the condition is true
        }
    }
}

• if...else

class Test {
    public static void main(String[] args) {
        if ( /* condition */ ) {
            // Code executed when the condition is true
        } else {
            // Code executed when the condition is false
        }
    }
}

• if...else if...else

class Test {
    public static void main(String[] args) {
        if ( /* condition */ ) {
            // Code executed when the condition is true
        } else if ( /* condition 2 */ ) {
            // Code executed when condition 2 is true
        } else {
          // Code executed when none of the above conditions is true
        }
    }
}

• switch...case

class Test {
    public static void main(String[] args) {
        switch ( /* expression */ ){
          case /* value 1 */:
              // Execute this code when the expression's value matches value 1
              break; // Without break, the program continues executing in order until it reaches break
          case /* value 2 */:
              // Execute this code when the expression's value matches value 2
              break;
          default:
              // Execute this code when the expression does not match any value listed above
        }
    }
}

Loops

• for

The for keyword has two uses. The first is the ordinary for loop, whose form is as follows:

class Test {
    public static void main(String[] args) {
        for ( /* initialization */; /* loop condition */; /* step executed after each loop */ ) {
            // Code in the loop body executed when the loop condition is true
        }
    }
}

The second is a use similar to C++'s foreach, for iterating over data in an array or collection. It is equivalent to hiding the loop variable in the previous form:

class Test {
    public static void main(String[] args) {
        for ( /* element type X */ /* element name Y */ : /* collection Z */ ) {
            // On each iteration of this block, element Y is one element of collection Z.
        }
    }
}

• while

class Test {
    public static void main(String[] args) {
        while ( /* condition */ ) {
            // Code in the loop body executed when the condition is true
        }
    }
}

• do...while

class Test {
    public static void main(String[] args) {
        do {
          // Code to execute
        } while ( /* loop condition */ );
    }
}

Notes

Class name and file name must match

When creating a Java source program, the class name and file name must match for compilation to succeed; otherwise, the compiler will report that it cannot find the class. Usually, the file name is specified by the particular OJ.

Example:

Add.java

class Add {
    public static void main(String[] args) {
        // ...
    }
}

The class in this file must be named Add for compilation to succeed.