Java面向对象（1）

static静态变量

public class Student {static String name;private double score;public Student(){};public Student(double score) {this.score = score;}public double getScore() {return score;}public void setScore(double score) {this.score = score;}
}

public class test {public static void main(String[] args) {Student.name="刘";Student s1=new Student();s1.name="马";Student s2=new Student();s2.name="秋";System.out.println(Student.name);}
}//秋

static只有一个变量，所以修改别的类中的变量，所以的name都会变化。

应用场景

比如系统的中统计用户的个数，在每次用户类中的构造函数中++。

public class User {public static int num;//初始为0public User(){User.num+=1;}
}

public class test {public static void main(String[] args) {User u1=new User();User u2=new User();User u3=new User();User u4=new User();User u5=new User();User u6=new User();System.out.println(u1.num);}
}
//输出 6

public class LoginDemo {public static void main(String[] args) {String code="";String data="qwertyuiopasdfghjklzxcvbnm";Random r=new Random();for(int i=0;i<4;++i){int t=r.nextInt(data.length());code+=data.charAt(t);}System.out.println(code);}
}

public class RegisterDemo {public static void main(String[] args) {String code="";String data="qwertyuiopasdfghjklzxcvbnm";Random r=new Random();for(int i=0;i<4;++i){int t=r.nextInt(data.length());code+=data.charAt(t);}System.out.println(code);}
}

上述两个代码可以改成如下：

public class MyUtil {public static String createCode(int n) {String code="";String data="qwertyuiopasdfghjklzxcvbnm";Random r=new Random();for(int i=0;i<n;++i){int t=r.nextInt(data.length());code+=data.charAt(t);}return code;}
}

public class LoginDemo {public static void main(String[] args) {System.out.println(MyUtil.createCode(4));}
}

public class RegisterDemo {public static void main(String[] args) {System.out.println(MyUtil.createCode(5));}
}

这样就会简洁很多。
在MyUtil中加入private MyUtil(){};后，这样可以防止别的代码中创建MyUtil类。

静态代码块

public class Student {static int num;static {System.out.println("静态代码块执行了");}
}

public class Test {public static void main(String[] args) {System.out.println(Student.num);System.out.println(Student.num);}
}
//输出
//静态代码块执行了
//0
//0

静态代码块只有在创建的第一次会执行。一般用来对类变量进行初始化赋值。

实例代码块

public class Student {public Student(){System.out.println("无参构造函数执行了");}public Student(String name){System.out.println("有参构造函数执行了");}{System.out.println("实例代码块执行了");}
}

public class Test {public static void main(String[] args) {Student stu=new Student();}
}
//输出
实例代码块执行了
无参构造函数执行了

单例模式

饿汉式单例
在还没有创建对象的时候就自己创建了

public class A {private static A a=new A();private A(){}public static A getObject(){return a;}
}

public class Test {public static void main(String[] args) {A a1=A.getObject();A a2=A.getObject();System.out.println(a1);System.out.println(a2);}
}
//输出
com.peng.test.single.A@4eec7777
com.peng.test.single.A@4eec7777

懒汉式单例
只有创建对象的时候才会创建，并且只能有一个对象

//懒汉式单例
public class B {private static B b;private B(){}public static B getInstance(){if(b==null){b = new B();}return b;}
}

public class Test {public static void main(String[] args) {B b1=B.getInstance();B b2=B.getInstance();System.out.println(b1);System.out.println(b2);}
}
//输出
com.peng.test.single.B@4eec7777
com.peng.test.single.B@4eec7777

继承

继承不能继承多个，但是可以多层。
任何类都默认继承Object类，

public class A {public int i;public void printi(){System.out.println("公共的可以继承");}private int j;private void printj(){System.out.println("私有的不可以继承");}}

public class B extends A {public void printb(){i=0;System.out.println(i);printi();}
}

继承的应用

public class people {private String name;public String getName() {return name;}public void setName(String name) {this.name = name;}
}

public class teacher {private int salary;public int getSalary() {return salary;}public void setSalary(int salary) {this.salary = salary;}
}

public class student {private String subject;public String getSubject() {return subject;}public void setSubject(String subject) {this.subject = subject;}
}

public class Test {public static void main(String[] args) {teacher t=new teacher();t.setName("刘");t.setSalary(100);System.out.println(t.getName());System.out.println(t.getSalary());}}
//输出
刘
100

修饰符

方法重写

重写的前面要加上@Override,便于理解。
继承变量中会就近原则。如果非要访问父类的变量，就用super来访问。

public class people {private String name;public String getName() {return name;}public void setName(String name) {this.name = name;}public void print_people(){System.out.println("1111");}
}

public class teacher extends people{private int salary;public int getSalary() {return salary;}public void setSalary(int salary) {this.salary = salary;}@Overridepublic void print_people(){System.out.println("66666");}
}

public class Test {public static void main(String[] args) {teacher t=new teacher();t.print_people();}
}