02java基础

约定不等于承诺〃 2022-01-09 12:51 244阅读 0赞

## 1.类的定义 ##

/*
        定义类:
          使用类的形式,对现实中的事物进行描述
          事物: 属性，方法
            属性: 变量
            方法: 这个事物具备的功能
    
         格式:
           public class 类名{
                属性定义
                  修饰符 数据类型 变量名 = 值
                
                方法定义
                  修饰符 返回值类型  方法名(参数列表){
                      
                  }
           }
           
    */

#Car.java
    package cn.jxufe.java.chapter2;
    
    /*
     *  类的方式,描述现实中的事物 小汽车
     *  
     *    小汽车  属性和功能
     *      属性: 颜色  轮胎个数    变量定义
     *      功能: 跑  方法
     *      
     *    属性和方法,都属于类的成员
     *    
     *    属性, 成员变量
     *    方法, 成员方法
     */
    public class Car {
        // 定义Car类的属性
    
        // 定义颜色属性
        String color;
        // 定义轮胎个数
    
        int count;
    
        // 定义跑的功能
        public void run() {
            System.out.println("小汽车在跑 ..." + color + "..." + count);
        }
    
    }

#TestCar.java
    package cn.jxufe.java.chapter2;
    
    public class TestCar {
    
        public static void main(String[] args) {
            // TODO Auto-generated method stub
            Car c = new Car();
            // 对象.调用类中的属性和方法
            c.color = "无色";
            c.count = 5;
            c.run();
        }
    
    }

![1542450-20190521104213378-951976310.png][]

![1542450-20190521104619147-205353358.jpg][]

## 2.成员变量和局部变量对比 ##

/*
     *  成员变量和局部变量的区别
     *  
     *  1. 定义位置上的区别
     *    成员变量,定义在类中,方法外
     *    局部变量,方法内,语句内
     *    
     *  2. 作用域不同
     *    成员变量,作用范围是整个类
     *    局部变量,方法内,语句内
     *    
     *  3. 默认值不同
     *     成员变量,有自己的默认值
     *     局部变量,没有默认值,不赋值不能使用
     *     
     *  4. 内存位置不同
     *     成员变量,跟随对象进入堆内存存储
     *     局部变量,跟随自己的方法,进入栈内存
     *     
     *  5. 生命周期不同
     *     成员变量,跟随对象,在堆中存储,内存等待JVM清理 , 生命相对较长
     *     局部变量,跟随方法,方法出栈    生命相对较短
     */

## 3.方法参数是基本数据类型和引用数据类型的区别 ##

#Person.java
    package cn.jxufe.java.chapter2.demo2;
    
    public class Person {
        String name;
    }

# TestPerson
    package cn.jxufe.java.chapter2.demo2;
    
    /*
     *  方法的参数类型,是基本数据类型,引用数据类型
     */
    public class TestPerson {
        public static void main(String[] args) {
            int a = 1;
            function(a);
            System.out.println(a);
    
            Person p = new Person();
            p.name = "张三";
            System.out.println(p.name);
    
            function(p);
    
            System.out.println(p.name);
        }
    
        /*
         * 定义方法,参数类型是引用数据类型 参数是Person类型 p接受的是一个内存的地址 main 中的
         * 变量p function 中的变量p 保存的地址是一样的
         */
        public static void function(Person p) {
            p.name = "李四";
        }
    
        /*
         * 定义方法,参数类型是基本数据类型
         */
        public static void function(int a) {
            a += 5;
        }
    }

![1542450-20190521123809080-506156096.png][]

## 4.实例变量和类变量、常量、方法 ##

![1542450-20190521151512425-732917356.png][]

![1542450-20190521164942930-1924496753.png][]![1542450-20190521165025348-1650270849.png][]

![1542450-20190521165158741-2088580978.png][]

package cn.jxufe.java.chapter2.demo8;
    
    public class TestCircleWithStaticMembers {
    
        public static void main(String[] args) {
            // TODO Auto-generated method stub
            System.out.println("before creating objects");
            System.out.println("the number of Circle objects is " + CircleWithStaticMembers.numberOfObjects);
    
            CircleWithStaticMembers c1 = new CircleWithStaticMembers();
    
            System.out.println("\nAfter creating c1");
            System.out.println("c1: radius " + c1.radius + " and number of Circle objects " + c1.numberOfObjects);
            
            CircleWithStaticMembers c2 = new CircleWithStaticMembers(5);
            
            System.out.println("\nAfter creating c2");
            System.out.println("c1: radius " + c1.radius + " and number of Circle objects " + c1.numberOfObjects);
            System.out.println("c2: radius " + c2.radius + " and number of Circle objects " + c2.numberOfObjects);
        }
    
    }

![1542450-20190521182142100-874920531.png][]

![1542450-20190521182352284-931027207.png][]

![1542450-20190521183151422-327572517.png][]

**使用下面的方法进行改正**

package cn.jxufe.java.chapter2.demo9;
    
    public class StaticMethod {
        int i = 5;
        static int k = 2;
    
        public static void main(String[] args) {
            StaticMethod a = new StaticMethod();
            int j = a.i; 
            a.m1(); 
        }
    
        public void m1() {
            i = i + k + m2(i, k);
            System.out.println("i = " + i);
        }
    
        public static int m2(int i, int j) {
            return (int) (Math.pow(i, j));
        }
    
    }

![1542450-20190521183526357-882130324.png][]

## 5.构造方法 ##

![1542450-20190521152542741-465118124.png][]

#SimpleCircle
    package cn.jxufe.java.chapter2.demo3;
    
    public class SimpleCircle {
        double radius;
    
        public SimpleCircle() {
            // TODO Auto-generated constructor stub
            radius = 1;
        }
    
        public SimpleCircle(double radius) {
            this.radius = radius;
        }
    
        public void setRadius(double radius) {
            this.radius = radius;
        }
    
        double getArea() {
            return radius * radius * Math.PI;
        }
    
        double getPerimeter() {
            return 2 * radius * Math.PI;
        }
    }

#TestSimpleCircle
    package cn.jxufe.java.chapter2.demo3;
    
    public class TestSimpleCircle {
    
        public static void main(String[] args) {
            // TODO Auto-generated method stub
            SimpleCircle circle1 = new SimpleCircle();
            System.out.println("the area of the circle of radius " + circle1.radius + " is " + circle1.getArea());
    
            SimpleCircle circle2 = new SimpleCircle(25);
            System.out.println("the area of the circle of radius " + circle2.radius + " is " + circle2.getArea());
    
            SimpleCircle circle3 = new SimpleCircle(125);
            System.out.println("the area of the circle of radius " + circle3.radius + " is " + circle3.getArea());
        }
    
    }

![1542450-20190521153826344-2009298699.png][]

## 6.private关键字 ##

描述人。Person

属性：年龄。

行为：说话：说出自己的年龄。

#Person.java
    package cn.jxufe.java.chapter2.demo4;
    
    public class Person {
        int age;
        String name;
    
        public  void show() {
            System.out.println(" age= " + age + " name " + name);
        }
    }

#TestPerson.java
    package cn.jxufe.java.chapter2.demo4;
    
    public class TestPerson {
        public static void main(String[] args) {
            // 创建Person对象
            Person p = new Person();
            p.age = -20; // 给Person对象赋值
            p.name = "人妖";
            p.show(); // 调用Person的show方法
        }
    }

![1542450-20190521155431523-1766580325.png][]

通过上述代码发现，虽然我们用Java代码把Person描述清楚了，但有个严重的问题，就是Person中的属性的行为可以任意访问和使用。这明显不符合实际需求。

可是怎么才能不让访问呢？需要使用一个Java中的关键字也是一个修饰符 private(私有，权限修饰符)。只要将Person的属性和行为私有起来，这样就无法直接访问。

#Person.java
    package cn.jxufe.java.chapter2.demo5;
    
    public class Person {
        private int age;
        String name;
    
        // private是成员修饰符，不能修饰局部变量，被private修饰的成员变量，只能在该类的内容使用，超出该类的范围都不能使用。
        public void setAge(int age) {
            if (age < 0 || age > 130) {
                System.out.println(age + " 不符合年龄范围");
                return;
            } else {
                this.age = age;
            }
        }
    
        public void show() {
            System.out.println(" age= " + age + " name " + name);
        }
    }

#TestPerson.java
    package cn.jxufe.java.chapter2.demo5;
    
    import cn.jxufe.java.chapter2.demo5.Person;
    
    public class TestPerson {
        public static void main(String[] args) {
            // 创建Person对象
            Person p = new Person();
            p.setAge(-20); // 给Person对象赋值
            p.name = "人妖";
            p.show(); // 调用Person的show方法
        }
    }

总结：

类中不需要对外提供的内容都私有化，包括属性和方法。

以后再描述事物，属性都私有化，并提供setXxx getXxx方法对其进行访问。

注意：私有仅仅是封装的体现形式而已。

## 7.this关键字 ##

**局部变量和成员变量同名问题**

不加this，就近原则调用

package cn.jxufe.java.chapter2.demo6;
    
    public class Person {
        int age;
        public void speak() {
            int age = 18;
            System.out.println("age = " + age);
        }
    }

package cn.jxufe.java.chapter2.demo6;
    
    public class TestPerson {
    
        public static void main(String[] args) {
            // TODO Auto-generated method stub
            Person person = new Person();
            person.age = 28;
            person.speak();
            System.out.println("person.age= " + person.age);
        }
    
    }

![1542450-20190521161830091-2092111604.png][]

加this，指明了调用的是成员变量

package cn.jxufe.java.chapter2.demo6;
    
    public class Person {
        int age;
        public void speak() {
            int age = 18;
            System.out.println("age = " + this.age);
        }
    }

![1542450-20190521161921219-1197812131.png][]

**this哪个对象调用了this所在的方法，this就代表哪个对象。**

## 8.访问权限 ##

![1542450-20190521162816375-1978211788.png][]![1542450-20190521162820659-416081516.png][]

## 9.java库中的类 ##

### 9.1Date类 ###

![1542450-20190521163536273-470943908.png][]

package cn.jxufe.java.chapter2.demo7;
    
    import java.util.Date;
    
    public class DateTest {
    
        public static void main(String[] args) {
            // TODO Auto-generated method stub
            Date date = new Date();
            System.out.println("the elapsed times since Jan 1,1970 is " + date.getTime() + " milliseconds");
            System.out.println(date.toString());
        }
    
    }

![1542450-20190521163610283-1024093197.png][]

### 9.2Random类 ###

![1542450-20190521163733334-1437256454.png][]

package cn.jxufe.java.chapter2.demo7;
    
    import java.util.Random;
    
    public class ch02RandomTest {
    
        public static void main(String[] args) {
            // TODO Auto-generated method stub
            Random random1 = new Random(3);
            System.out.println("from random1:");
            for (int i = 0; i < 10; i++) {
                System.out.print(random1.nextInt(10)+ " ");
            }
    
            Random random2 = new Random(3);
            System.out.println("\nfrom random2:");
            for (int i = 0; i < 10; i++) {
                System.out.print(random2.nextInt(10) + " ");
            }
        }
    
    }

![1542450-20190521164201524-650355490.png][]

## 10.面向对象的简单应用 ##

### 10.1贷款的例子 ###

![1542450-20190521184530475-599193512.png][]![1542450-20190521184604013-1008336280.png][]

package cn.jxufe.java.chapter2.demo10;
    
    import java.util.Date;
    
    public class Loan {
        private double annuInterestRate;
        private int numberOfYears;
        private double loanAmount;
        private Date loanDate;
    
        public Loan() {
            // TODO Auto-generated constructor stub
            this(2.5, 1, 1000); // 相当于直接调用下面的构造方法
        }
    
        public Loan(double annuInterestRate, int numberOfYears, double loanAmount) {
            this.annuInterestRate = annuInterestRate;
            this.numberOfYears = numberOfYears;
            this.loanAmount = loanAmount;
            loanDate = new Date();
        }
    
        public double getAnnuInterestRate() {
            return annuInterestRate;
        }
    
        public void setAnnuInterestRate(double annuInterestRate) {
            this.annuInterestRate = annuInterestRate;
        }
    
        public int getNumberOfYears() {
            return numberOfYears;
        }
    
        public void setNumberOfYears(int numberOfYears) {
            this.numberOfYears = numberOfYears;
        }
    
        public double getLoanAmount() {
            return loanAmount;
        }
    
        public void setLoanAmount(double loanAmount) {
            this.loanAmount = loanAmount;
        }
    
        public Date getLoanDate() {
            return loanDate;
        }
    
        public void setLoanDate(Date loanDate) {
            this.loanDate = loanDate;
        }
    
        public double getMonthlyPayment() {
            double monthlyInterestRate = annuInterestRate / 1200;
            double monthlyPayment = loanAmount * monthlyInterestRate
                    / (1 - (1 / Math.pow(1 + monthlyInterestRate, numberOfYears * 12)));
            return monthlyPayment;
        }
    
        public double getTotalPayment() {
            double totalPayment = getMonthlyPayment() * numberOfYears * 12;
            return totalPayment;
        }
    
    }

package cn.jxufe.java.chapter2.demo10;
    
    import java.util.Scanner;
    
    public class TestLoan {
    
        public static void main(String[] args) {
            // TODO Auto-generated method stub
            Scanner input = new Scanner(System.in);
            System.out.println("enter annual interest rate,for example,8.25:");
            double annualInterestRate = input.nextDouble();
    
            System.out.println("enter number of years as an integer: ");
            int numberOfYears = input.nextInt();
    
            System.out.println("enter loan amount,for example,12000: ");
            double loanAmount = input.nextDouble();
    
            Loan loan = new Loan(annualInterestRate, numberOfYears, loanAmount);
            
            //注意此处的 printf
            System.out.printf("the loan was created on %s\n " + "the monthly payment is %.2f\n The total paymen is %.2f\n",
                    loan.getLoanDate().toString(), loan.getMonthlyPayment(), loan.getTotalPayment());
    
        }
    
    }

![1542450-20190521191458965-2045469532.png][]

### 10.2设计栈类 ###

![1542450-20190521191938584-2105871906.png][]

package cn.jxufe.java.chapter2.demo11;
    
    public class StackOfInteger {
        private int[] elements;
        private int size;
        public static final int DEFAULT_CAPACITY = 16;
    
        public StackOfInteger(int capacity) {
            // TODO Auto-generated constructor stub
            elements = new int[capacity];
        }
    
        public StackOfInteger() {
            this(DEFAULT_CAPACITY);
        }
    
        public void push(int value) {
            if (size >= elements.length) {
                int[] temp = new int[elements.length * 2];
                System.arraycopy(elements, 0, temp, 0, elements.length);
                elements = temp;
            }
            elements[size++] = value;
        }
    
        public int pop() {
            return elements[--size];
        }
    
        public int getSize() {
            return size;
        }
    
        public boolean empty() {
            return size == 0;
        }
    }

package cn.jxufe.java.chapter2.demo11;
    
    public class TestStackOfInteger {
    
        public static void main(String[] args) {
            // TODO Auto-generated method stub
            StackOfInteger stack = new StackOfInteger();
    
            for (int i = 0; i < 10; i++) {
                stack.push(i);
            }
            while (!stack.empty()) {
                System.out.print(stack.pop() + " ");
            }
        }
    
    }

![1542450-20190521193506098-218763186.png][]

转载于:https://www.cnblogs.com/xinmomoyan/p/10901882.html

[1542450-20190521104213378-951976310.png]: /images/20220109/e1afaac77f3f49dd855684a9ebfa6208.png
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