Java学习笔记

Java是一种跨平台的编程语言，由于其特有的“一次编写，到处运行”的特点，在当今软件开发领域应用十分广泛。而廖雪峰Java教程从Java的基础入手，逐步深入到高级应用，是学习Java的好帮手。

一、Java基础

1、变量、数据类型

public class HelloWorld {
    public static void main(String[] args) {
        int x = 5;
        double y = 3.14;
        String s = "Hello World!";
        System.out.println(x);
        System.out.println(y);
        System.out.println(s);
    }
}

2、数组、字符串

public class Array {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3};
        String s = "Hello World!";
        System.out.println(arr[1]);
        System.out.println(s.substring(0, 5));
    }
}

3、流程控制语句

public class FlowControl {
    public static void main(String[] args) {
        int score = 80;
        if (score >= 90) {
            System.out.println("优秀");
        } else if (score >= 80) {
            System.out.println("良好");
        } else if (score >= 60) {
            System.out.println("及格");
        } else {
            System.out.println("不及格");
        }
    }
}

二、面向对象编程

1、类、对象

public class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public void sayHello() {
        System.out.println("Hello, my name is " + name + ", I'm " + age + " years old.");
    }
}

public class Test {
    public static void main(String[] args) {
        Person p = new Person("Tom", 20);
        p.sayHello();
    }
}

2、继承、多态

public abstract class Animal {
    public abstract void shout();
}

public class Dog extends Animal {
    @Override
    public void shout() {
        System.out.println("wangwang");
    }
}

public class Cat extends Animal {
    @Override
    public void shout() {
        System.out.println("miaomiao");
    }
}

public class Test {
    public static void main(String[] args) {
        Animal a1 = new Dog();
        Animal a2 = new Cat();
        a1.shout();
        a2.shout();
    }
}

3、接口、内部类

public interface Shape {
    double area();
}

public class Circle implements Shape {
    private double r;

    public Circle(double r) {
        this.r = r;
    }

    @Override
    public double area() {
        return Math.PI * r * r;
    }

    public static class InnerClass {
        public void sayHi() {
            System.out.println("Hi");
        }
    }
}

public class Test {
    public static void main(String[] args) {
        System.out.println(new Circle(5).area());
        new Circle.InnerClass().sayHi();
    }
}

三、异常处理

1、异常分类

public class ExceptionTest {
    public static void main(String[] args) {
        int[] arr = new int[3];
        try {
            arr[4] = 5;
        } catch (ArrayIndexOutOfBoundsException e) {
            System.out.println("数组越界");
        }
    }
}

2、自定义异常

public class MyException extends Exception {
    public MyException(String message) {
        super(message);
    }
}

public class ExceptionTest {
    public static void main(String[] args) {
        try {
            throw new MyException("发生了自定义异常");
        } catch (MyException e) {
            System.out.println(e.getMessage());
        }
    }
}

3、finally语句块

public class ExceptionTest {
    public static void main(String[] args) {
        FileInputStream fis = null;
        try {
            fis = new FileInputStream("test.txt");
            //...
        } catch (FileNotFoundException e) {
            System.out.println("文件不存在");
        } finally {
            if (fis != null) {
                try {
                    fis.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

四、Java IO及文件操作

1、输入输出流

public class FileCopy {
    public static void main(String[] args) {
        FileInputStream fis = null;
        FileOutputStream fos = null;
        try {
            fis = new FileInputStream("src.txt");
            fos = new FileOutputStream("dest.txt");
            byte[] buffer = new byte[1024];
            int len;
            while ((len = fis.read(buffer)) != -1) {
                fos.write(buffer, 0, len);
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (fis != null) {
                try {
                    fis.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (fos != null) {
                try {
                    fos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

2、字符输入输出流

public class FileCopy {
    public static void main(String[] args) {
        InputStreamReader isr = null;
        OutputStreamWriter osw = null;
        try {
            isr = new InputStreamReader(new FileInputStream("src.txt"), "UTF-8");
            osw = new OutputStreamWriter(new FileOutputStream("dest.txt"), "UTF-8");
            char[] buffer = new char[1024];
            int len;
            while ((len = isr.read(buffer)) != -1) {
                osw.write(buffer, 0, len);
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (isr != null) {
                try {
                    isr.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (osw != null) {
                try {
                    osw.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

3、文件操作

public class FileOperation {
    public static void main(String[] args) {
        File dir = new File("/Users/liuyang/Desktop");
        if (!dir.exists() || !dir.isDirectory()) {
            System.out.println("不存在该目录");
            return;
        }
        File[] files = dir.listFiles();
        for (File file : files) {
            if (file.isFile()) {
                System.out.println(file.getName());
            }
        }
    }
}