学Java第二十六天---------static的用法

1.静态变量static

1.1什么是static

1.2什么是静态变量

1.3静态变量代码示例:

public class Student {
    private String name;
    private int age;
    private String id;
    static String teacherName;

    public Student() {
    }

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

比如Student类,有静态变量teacherName

教师是被所有学生共享的,所以teacherName可以作为静态变量

用static修饰,则该变量就可以被所有对象共享,所有对象的teacherName都是同一个值

而且成员变量默认是public

---

Student.teacherName="阿伟老师";

可以先于对象被创建

public class Studenttset {
    public static void main(String[] args) {
        Student.teacherName="张老师";
        Student s1=new Student("小红",21,"heima001");
        System.out.println(s1.teacherName);
    }
}

2.静态方法

在本类中,直接方法名就能调用,要调用其他类的方法,则要用类名调用

 工具类:

2.1.什么是工具类,有什么作用

2.2.工具类如何构造

2.3工具类相关练习

2.3.1定义数组工具类

工具类代码:

package com.itbaima.exe05;

public class ArrayUtil {
    //构造方法一定是私有的,目的是防止创建对象
    private ArrayUtil(){}

    public static String printArr(int []arr){
        StringBuilder sb=new StringBuilder();
        sb.append("[");
        for (int i = 0; i < arr.length; i++) {
            if(i==arr.length-1){
                sb.append(arr[i]);
            }
            else{
                sb.append(arr[i]).append(",");
            }
        }
        sb.append("]");
        return sb.toString();
    }

    public static double getAverage(double arr[]){
        double sum=0;
        for (int i = 0; i < arr.length; i++) {
            sum=sum+arr[i];
        }
        return sum/arr.length;
    }
}

测试类代码:

public class TestDemo {
    public static void main(String[] args) {
        int arr[]={1,2,3,4,5};
        String str=ArrayUtil.printArr(arr);
        System.out.println(str);


        double []arr2={5.1,4.6,8.3,4.9,1.3};
        double result=ArrayUtil.getAverage(arr2);
        System.out.println(result);
    }

2.3.2定义学生工具类

JavaBean类代码:

package com.itbaima.exe06;

public class Student {
    private String name;
    private int age;
    private String gender;

    public Student() {
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;
    }
}

工具类代码:

package com.itbaima.exe06;

import java.util.ArrayList;

public class StudentUtil {
    private StudentUtil(){}

    public static int maxstudentAge(ArrayList<Student> list){
        int max=list.get(0).getAge();

        for (int i = 0; i < list.size(); i++) {
            if(list.get(i).getAge()>max){
                max=list.get(i).getAge();
            }
        }
        return max;
    }
}

测试类代码:

package com.itbaima.exe06;

import java.util.ArrayList;

public class Test {
    public static void main(String[] args) {
        //1.先创建一个集合
        ArrayList<Student> list=new ArrayList<>();

        //2.再创建三个学生对象
        Student s1= new Student("xiaohong",21,"男");
        Student s2= new Student("xiaolv",22,"女");
        Student s3= new Student("xiaolan",21,"男");

        //3.将三个学生加入集合中
        list.add(s1);
        list.add(s2);
        list.add(s3);

        int age=StudentUtil.maxstudentAge(list);

        System.out.println(age);
    }
}

3.静态static的注意事项