第三章：常用api

一、api的使用

快捷键的话必须先完整输入方法 例如：sc.nextLine()

二、String构造方法

public class Demo {
    public static void main(String[] args) {
        String sr = new String();
        System.out.println("String是："+sr);
        char[] ch = {'a','b','c'};
        String s2 = new String(ch);
        System.out.println("char是："+s2);
        byte[] by = {60, 70, 80, 90};
        String s3 = new String(by);
        System.out.println("byte是："+s3);
        String s4 = "abcd";
        System.out.println("String2是："+s4);
    }
}

三、String字符串的比较

public class Demo {
    public static void main(String[] args) {
        char[] ch = {'a','b','c'};
        String s1 = new String(ch);
        System.out.println("s1是："+s1);
        String s2 = new String(ch);
        System.out.println("s2是："+s2);
        String s3 = "abc";
        System.out.println("String3是："+s3);
        String s4 = "abc";
        System.out.println("String4是："+s4);
        String print = "";
        for (int a=0;a<20;a++){
            print += "-";
        }
        System.out.println(print);
        System.out.println(s1 == s2);
        System.out.println(s3 == s4);
        System.out.println(s1 == s3);
        System.out.println(s1.equals(s2));
        System.out.println(s3.equals(s4));
        System.out.println(s1.equals(s3));
    }
}

1. 用户登录判断案例

import java.util.Scanner;
public class Demo {
    public static void main(String[] args) {
        String name = "cao";
        String pwd = "123";
        Scanner sc = new Scanner(System.in);
        for (int i = 0;i < 3;i++){
            System.out.println("请输入用户名");
            String input_name = sc.nextLine();
            System.out.println("请输入密码");
            String input_pwd = sc.nextLine();
            if (name.equals(input_name) && pwd.equals(input_pwd)){
                System.out.println("登录成功");
                break;
            }else if(2-i == 0){
                System.out.println("登录失败，账户被锁定");
                break;
            }else{
                System.out.print("登陆失败，你还有" + (2 - i) + "次机会");
            }
        }
    }
}

2. 遍历字符串

import java.util.Scanner;
public class Demo {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("请输入一个字符串：");
        String text = sc.nextLine();
        for (int i=0;i<text.length();i++){
            System.out.println(text.charAt(i));
        }
    }
}

3. 统计字符次数

import java.util.Scanner;
public class Demo {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("请输入一个字符串：");
        String text = sc.nextLine();
        int big=0;
        int small=0;
        int number=0;
        for (int i = 0;i<text.length();i++){
            if (text.charAt(i)>='a' && text.charAt(i)<='z'){
                small++;
            }else if(text.charAt(i)>='A' && text.charAt(i)<='Z'){
                big++;
            }else if (text.charAt(i)>='0' && text.charAt(i)<='9'){
                number++;
            }
        }
        System.out.println("big数量："+big);
        System.out.println("small数量："+small);
        System.out.println("number数量："+number);
    }
}

4. 字符串拼接

public class Demo {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3};
        String a = arr_join(arr);
        System.out.println(a);
    }
    public static String arr_join(int[] arr){
        String s = "";
        s += "[";
        for (int a=0;a<arr.length;a++){
            if (a == arr.length-1){
                s += arr[a];
                s += ']';
            }else{
                s += arr[a];
                s += ", ";
            }
        }
        return s;
    }
}

三、StringBuilder和String

1. 相互转换

import java.util.Scanner;
public class Demo {
    public static void main(String[] args) {
        String s = "hello";
        StringBuilder st = new StringBuilder(s);
        System.out.println(st);
        StringBuilder st2 = new StringBuilder();
        st2.append("world");
        String s2 = st2.toString();
        System.out.println(s2);
    }
}

2. 字符串的拼接

import java.util.Scanner;
public class Demo {
    public static void main(String[] args) {
        int[] arr = {1,2,3,4};
        String s = arrayToString(arr);
        System.out.println(s);
    }
    public static String arrayToString(int[] arr){
        StringBuilder st = new StringBuilder();
        st.append("[");
        for (int i=0;i<arr.length;i++){
            if (i == arr.length-1){
                st.append(arr[i]);
            }else {
                st.append(arr[i]).append(", ");
            }
        }
        st.append("]");
        String s = st.toString();
        return s;
    }
}

3. 字符串反转

import java.util.Scanner;
public class Demo {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("请输入一个字符：");
        String line = sc.nextLine();
        String result = myReversal(line);
        System.out.println(result);
    }
    public static String myReversal(String s){
        StringBuilder st = new StringBuilder(s);
        st.reverse();
        String s2 = st.toString();
        return s2;
    }
}

四、ArrayList集合

1. 集合概述

2. ArrayList方法添加和使用

import java.util.ArrayList;
public class Demo {
    public static void main(String[] args) {
        ArrayList<String> ar = new ArrayList<>();
        ar.add("hello");
        ar.add("world");
        ar.add("java");
        ar.add(1,"javaee");
        System.out.println(ar);
    }
}

3. ArrayList常用方法

4. ArrayList存储和遍历

import java.util.ArrayList;
public class Demo {
    public static void main(String[] args) {
        ArrayList<String> arr = new ArrayList<>();
        for (int i=0;i<5;i++){
            arr.add("这是集合"+i);
        }
        System.out.println("该数组集合长度是："+arr.size());
        for(int i = 0;i<arr.size();i++){
            String s = arr.get(i);
            System.out.println("数组"+i+"是："+s);
        }
    }
}

5. 案例学生存储遍历

--------------------------------------------------------Student类
package com;
public class Student {
    private String gender;
    private String age;
    public Student(){}
    public Student(String gender, String age){
        this.gender = gender;
        this.age = age;
    }
    public void setgender(String gender) {
        this.gender = gender;
    }
    public void setAge(String age) {
        this.age = age;
    }
    public String getAge() {
        return age;
    }
    public String getgender() {
        return gender;
    }
    @Override
    public String toString() {
        return "Student{" +
                "gender='" + gender + '\'' +
                ", age=" + age +
                '}';
    }
}
--------------------------------------------------------Demo类
package com;
import java.util.ArrayList;
import java.util.Scanner;
public class Demo {
    public static void main(String[] args) {
        ArrayList<Student> arr = new ArrayList<>();
        for (int i = 0; i < 3; i++) {
            Student s1 = new Student();
            Scanner sc = new Scanner(System.in);
            System.out.println("请输入集合" + i + "的性别：");
            s1.setgender(sc.nextLine());
            System.out.println("请输入集合" + i + "的年龄：");
            s1.setAge(sc.nextLine());
            arr.add(s1);
        }
        for (int i = 0; i < arr.size(); i++) {
            String s = arr.get(i).toString();
            System.out.println(s);
        }
    }
}

五、Math

1. Math的常用方法

六、System

1. System的常用方法

七、Object类

1. toString

2. equals

3. 冒泡排序

import java.util.Arrays;
public class test {
    public static void main(String[] args) {
        int[] arr = {24,60,80,30,100,10};
        System.out.println("排序前：" + Arrays.toString(arr));
        for(int a=1; a<arr.length; a++){
            for(int i=0;i<arr.length-a;i++){
                if(arr[i]>arr[i+1]){
                    int temp = arr[i];
                    arr[i] = arr[i+1];
                    arr[i+1] = temp;
                }
            }
            System.out.println("第" + a + "次排序：" + Arrays.toString(arr));
        }
    }
}

4. Arrays类

八、基本类型包装类

1. 描述

项目

1. 案例学生管理系统

-----------------------------------------------------------------------Student类
package com;
import java.util.Scanner;
public class Student {
    private String name;
    private String address;
    private int age;
    private String sid;
    public Student(){}
    public Student(String name, String address, String sid, int age){
        this.name = name;
        this.address = address;
        this.sid = sid;
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public int getAge() {
        return age;
    }
    public String getAddress() {
        return address;
    }
    public String getSid() {
        return sid;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public void setName(String name) {
        this.name = name;
    }
    public void setAddress(String address) {
        this.address = address;
    }
    public void setSid(String sid) {
        this.sid = sid;
    }
}
-----------------------------------------------------------------------Demo类
package com;
import java.util.ArrayList;
import java.util.Scanner;
public class Demo {
    public static void main(String[] args) {
        ArrayList<Student> arr = new ArrayList<>();
        while (true) {
            System.out.println("--------欢迎来到学生管理系统--------");
            System.out.println("1.添加学生");
            System.out.println("2.删除学生");
            System.out.println("3.修改学生");
            System.out.println("4.查看所有学生");
            System.out.println("5.退出");
            System.out.println("请输入你的选择：");
            Scanner sc = new Scanner(System.in);
            String line = sc.nextLine();
            switch (line) {
                case "1":
                    addStudent(arr);
                    break;
                case "2":
                    deleteStudent(arr);
                    break;
                case "3":
                    amendStudent(arr);
                    break;
                case "4":
                    checkStudent(arr);
                    break;
                case "5":
                    System.out.println("谢谢使用");
                    System.exit(0);
            }
        }
    }
    public static void addStudent(ArrayList<Student> array) {
        int index = -1;
        Scanner sc = new Scanner(System.in);
        System.out.println("请输入学生学号：");
        String sid = sc.nextLine();
        for (int i=0;i<array.size();i++){
            if (array.get(i).getSid().equals(sid)){
                index = i;
                break;
            }
        }
        if (index != -1){
            System.out.println("你输入的学号存在，请重新输入");
            addStudent(array);
        }
        System.out.println("请输入学生姓名：");
        String name = sc.nextLine();
        System.out.println("请输入学生年龄：");
        int age = Integer.parseInt(sc.nextLine());
        System.out.println("请输入学生居住地：");
        String address = sc.nextLine();
        Student s = new Student();
        s.setSid(sid);
        s.setName(name);
        s.setAge(age);
        s.setAddress(address);
        array.add(s);
        System.out.println("添加学生成功");
    }
    public static void checkStudent(ArrayList<Student> array) {
        if (array.size() == 0){
            System.out.println("暂无成员，请先添加数据！");
            return;
        }
        System.out.println("学号\t\t\t姓名\t\t年龄\t\t住址");
        for (int i = 0; i < array.size(); i++) {
            String sid = array.get(i).getSid();
            String name = array.get(i).getName();
            int age = array.get(i).getAge();
            String address = array.get(i).getAddress();
            System.out.println(sid + "\t" + name + "\t" + age + "岁\t" + address);
        }
    }
    public static void deleteStudent(ArrayList<Student> array) {
        Scanner sc = new Scanner(System.in);
        System.out.println("请输入要删除的学号：");
        String sid = sc.nextLine();
        int index = -1;
        for (int i=0;i< array.size();i++){
            if (array.get(i).getSid().equals(sid)){
                index = i;
                break;
            }
        }
        if (index == -1){
            System.out.println("学号不存在，请重新输入");
            deleteStudent(array);
        }else{
            array.remove(index);
            System.out.println("成功删除一条信息！");
            checkStudent(array);
        }
    }
    public static void amendStudent(ArrayList<Student> array) {
        System.out.println("请输入要修改的学生的学号：");
        Scanner sc = new Scanner(System.in);
        String sid = sc.nextLine();
        int index = -1;
        for (int i=0;i< array.size();i++){
            if (array.get(i).getSid().equals(sid)){
                index = i;
                break;
            }
        }
        if (index == -1){
            System.out.println("学号不存在，请重新输入");
            amendStudent(array);
        }else{
            System.out.println("请输入学生的新学号");
            sid = sc.nextLine();
            array.get(index).setSid(sid);
            System.out.println("请输入学生的新姓名");
            String name = sc.nextLine();
            array.get(index).setName(name);
            System.out.println("请输入学生的新年龄");
            int age = Integer.parseInt(sc.nextLine());
            array.get(index).setAge(age);
            System.out.println("请输入学生的新地址");
            String address = sc.nextLine();
            array.get(index).setAddress(address);
            System.out.println("修改完毕");
            checkStudent(array);
        }
    }
}