Java笔记

变量

• 定义常量

final double PI=value

final int a=3

• 定义变量

adouble a = 12.2

public class zheng {
    public static void main(String args []){
        double a1=12.2;
        double s1=a1*a1;
        System.out.println("正方形的面积是"+s1);
    }
}

• 全局变量

public class quan {
    int z1 = 2;     //全局变量
    boolean e;
    public static void main(String [] args){
        int z1 = 111;
        System.out.println("打印数据z="+z1);
    }

}

• 局部变量

public class Test {
    public void pupAge(){
        int age = 0;         //局部变量
        age = age +7;
        System.out.println("小狗的年龄是: "+age);
    }
    public static void main(String[] args){
        Test test = new Test();
        test.pupAge();
    }
}

• 实例变量

1. 声明在类中，但在方法构造方法和语句块之外
2. 实例变量在对象创建的时候创建，在对象销毁的时候销毁
3. 实例变量具有默认值

import java.io.*;
public class Employee {
    public String name;      //实例变量，对子类可见
    public int age;                 //全局变量
    public String designation;
    private double salary;   //私有变量，仅在该类可见
    //Employee类的构造器
    public Employee(String empname){
        this.name = empname;
    }
    //设置age的值
    public void empAge(int empAge){
        this.age = empAge;
    }
    /*设置designation的值*/
    public void empDesignation(String empDesig){
        designation = empDesig;
    }
    /*设置salary的值*/
    public void empSalary(double empSalary){
        salary = empSalary;
    }
    /*打印信息*/
    public void printEmployee(){
        System.out.println("名字:"+name);
        System.out.println("年龄:"+age);
        System.out.println("职位:"+designation);
        System.out.println("薪水:"+salary);
    }
    public static void main(String[] args){
        Employee empOne = new Employee("RUNOOB");
        empOne.empSalary(1000.0);
        empOne.printEmployee();
    }
}

• 类变量

1. 常被用来申明为常量

import java.io.*;
 
public class Employee {
    //salary是静态的私有变量
    private static double salary;
    // DEPARTMENT是一个常量
    public static final String DEPARTMENT = "开发人员";
    public static void main(String[] args){
    salary = 10000;
        System.out.println(DEPARTMENT+"平均工资:"+salary);
    }
}

Java类和对象

public class Puppy {
    int puppyAge;
    public Puppy(String name){
        //这个构造器仅有一个参数
        System.out.println("小狗的名字是:"+name);
    }
    public void setAge(int age){
        puppyAge = age;
    }
    public int getAge(){
        System.out.println("小狗的年龄为: " + puppyAge);
        return puppyAge;
    }

    public static void main(String[] args){
        //下面的语句创建一个Puppy对象
        Puppy mypuppy = new Puppy("tommy");
        //通过方法设定age
        mypuppy.setAge(2);
        //调用另一个方法获取age
        mypuppy.getAge();
        //另一种方式访问成员变量
        System.out.println("变量值："+mypuppy.puppyAge);
    }
}

• 一个源文件中只允许有一个public类

• 文件名要与public类的名称相似

• import语句用来倒入java包

创建类：

Employee.java

import java.io.*;
public class Employee {
    String name;
    int age;
    String designation;
    double salary;
    //Employee类的构造器
    public Employee(String name){
        this.name = name;
    }
    //设置age的值
    public void empAge(int empAge){
        this.age = empAge;
    }
    /*设置designation的值*/
    public void empDesignation(String empDesig){
        designation = empDesig;
    }
    /*设置salary的值*/
    public void empSalary(double empSalary){
        salary = empSalary;
    }
    /*打印信息*/
    public void printEmployee(){
        System.out.println("名字:"+name);
        System.out.println("年龄:"+age);
        System.out.println("职位:"+designation);
        System.out.println("薪水:"+salary);
    }
}

调用类:

EmployeeTest.java

import java.io.*;
public class EmployeeTest {
    public static void main(String[] args){
        /*使用构造器创建两个对象*/
        Employee empOne = new Employee("RUNOOB1");
        Employee empTwo = new Employee("RUNOOB2");
        //调用两个对象成员的方法
        empOne.empAge(26);
        empOne.empDesignation("高级程序员");
        empOne.empSalary(1000);
        empOne.printEmployee();

        empTwo.empAge(21);
        empTwo.empDesignation("菜鸟程序员");
        empTwo.empSalary(500);
        empTwo.printEmployee();
    }
}