【笔记】Java从入门到精通 | 基础篇

如果你已经有了C语言或任何一门编程语言的基础（面向对象的更好），那么学习Java语言不难，特别是本文中的基础内容。找本书花一两天看看这部分内容，过一遍代码，其实就掌握了Java语言的基础，之后可以进一步学习Java面向对象的精髓，并且修炼Java编程思想，才能算是入了门道。

基础篇分为五个部分，第一部分“语言基础”和第二部分“流程控制”在所有编程语言中都差不多，这里不再赘述，只是关于常量和变量（包括static/final）的一些概念，以及循环控制中的foreach用法，简单提一下。

package com.kplayer
/*常量与变量*/
public class KPValue {
    static final double PI = 3.14; //静态常量，必须且只能一次赋值，可跨类调用
    final double PI2 = 3.14; //实例常量，必须创建实例对象，分配空间后使用
    
    static int a = 10;  //静态变量（类变量），属于类，加载后即可使用
    int b = 20; //实例变量，必须创建实例对象，分配空间后使用
    
    public static void main(String[] args) {
        int c = 30; //局部变量，且不能声明为static
        
        //PI = 3.1415;  //The final field KPValue.PI cannot be assigned
        //PI2 = 3.1415;  //Cannot make a static reference to the non-static field PI2
        a = 100;  //静态变量重赋值
        //b = 200; //必须创建实例对象，分配空间后使用
        c = 300;        
        System.out.println(a + "," + c);
    }
}

/*Foreach语句*/
public class KPForeach {
    public static void main(String args[]) {
        int arr[] = { 1, 3, 5 };
        for (int x : arr) {
            System.out.println(x);
        }
    }
}

第三部分“字符串”和第四部分“数组”是Java中重要的两个数据类型，需要掌握如何创建/初始化以及常用基本操作。

package com.kplayer

import java.util.Arrays;
import java.util.Date;

/*字符串*/
public class KPString {
    public static void main(String args[]) {
        String s1 = new String("hello");  //构造方法创建
        String s2 = "world ";  //字符串常量引用赋值创建
        String s3 = new String("a,b,c,d");
        int  year = 2018;
        
        //连接字符串
        String conc = s1 + " " + s2 + year;  //hello world 2018
        System.out.println(conc);
        
        //获取字符串信息
        int size = s1.length(); //5
        int indx = s1.indexOf("el"); //1
        int indxlast = s1.lastIndexOf("l"); //3
        char chr = s1.charAt(4); //o
        System.out.println(size + " " + indx + " " 
                + indxlast + " " + chr);
        
        //字符串操作
        String subs = s1.substring(0,2); //he
        String subs2 = s1.substring(2); //llo
        String trm = s2.trim(); //world
        String repl = s1.replace("h", "H"); //Hello
        boolean sta = s1.startsWith("he"); //true
        boolean end = s1.endsWith("O"); //false
        boolean equ = s1.equals(repl); //false
        boolean equ2 = s1.equalsIgnoreCase(repl); //true
        int com = s1.compareTo(s2); //-15
        String low = s1.toLowerCase(); //hello
        String up = s1.toUpperCase(); //HELLO
        System.out.println(subs + " " + subs2 + " " + 
                trm + " " + repl + " " + sta + " " + end + " " + 
                equ + " " + equ2 + " " + com + " " + low + " " + 
                up);
        String[] spl = s3.split(",",3); //a b c,d
        for(int i = 0; i < spl.length; i++){
            System.out.println(spl[i]);
        }
        
        //格式化
        Date date = new Date();
        String dat = String.format("%tF", date); //2018-07-21
        String time = String.format("%tc", date); //星期六 七月 21 09:42:44 CST 2018
        System.out.println(dat + ", " + time);
        String formt = String.format("%d", 400 / 2); //200
        String formb = String.format("%b", 3 > 5); //false
        String formx = String.format("%x", 200); //c8
        System.out.println(formt + " " + formb + " " + formx);
        
        //正则表达式
        String regex = "\w+@\w+(\.\w{2,3})*\.\w{2,3}";
        String str1 = "aaa@"; // 定义要进行验证的字符串
        String str2 = "aaaaa";
        String str3 = "1111@111ffyu.dfg.com";
        boolean match1 = str1.matches(regex); //false
        boolean match2 = str2.matches(regex); //false
        boolean match3 = str3.matches(regex); //true
        System.out.println(match1 + " " + match2 + " " + match3);
        
        //字符串生成器
        StringBuilder builder = new StringBuilder("");
        builder.append("Hello "); //Hello 
        builder.insert(6, "world Kplayer"); //Hello world Kplayer
        builder.delete(11, 19); //Hello world
        System.out.println(builder);    
    }
}

package com.kplayer

import java.util.Arrays;
import java.util.Date;

/*数组*/
public class KPArray {
    public static void main(String args[]) {
        //一维数组
        int arr[]; //声明
        arr = new int[5]; //分配内存
        int arr2[] = new int[5]; //声明时分配内存        
        int arr3[]=new int[]{1,2,3,4,5}; //初始化
        for (int i = 0; i < arr3.length; i++) {
            System.out.println(arr3[i]);
        }
        
        //二维数组
        int darr[][]; //声明
        darr = new int[2][4]; //分配内存
        //darr = new int[2][]; darr[0] = new int[2]; darr[1] = new int[3];
        int darr2[][] = new int[2][4]; //声明时分配内存    
        int darr3[][] = {{11,22,33,44},{21,22,23,24}};  //初始化
        
        //遍历
        for (int i = 0; i < darr3.length; i++) {
            for (int j = 0; j < darr3[i].length; j++) {
                System.out.print(darr3[i][j] + " ");
            }
            System.out.println();
        }
        for (int x[] : darr3) {
            for (int y : x) {
                System.out.print(y + " ");
            }
            System.out.println();
        }
        
        //填充
        int arrf[] = new int[]{1,2,3,4,5};
        Arrays.fill(arrf, 8); //8 8 8 8 8
        for (int i = 0; i < arrf.length; i++) {
            System.out.println(arrf[i]);
        }        
        Arrays.fill(arrf, 1, 3, 7); //8 7 7 8 8
        for (int i = 0; i < arrf.length; i++) {
            System.out.println(arrf[i]);
        }
        
        //排序
        int arrs[] = new int[]{20,18,7,21};
        Arrays.sort(arrs); //7 18 20 21
        for (int i = 0; i < arrs.length; i++) {
            System.out.println(arrs[i]);
        }        

        //复制
        int arrc[] = new int[]{20,18,7,21};
        int newarrc[] = Arrays.copyOf(arrc,5); //20 18 7 21 0
        for (int i = 0; i < newarrc.length; i++) {
            System.out.println(newarrc[i]);
        }    
        int newarrc2[] = Arrays.copyOfRange(arrc,0,2); //20 18
        for (int i = 0; i < newarrc2.length; i++) {
            System.out.println(newarrc2[i]);
        }
        
        //查询
        int arrb[] = new int[]{20,18,7,21};
        Arrays.sort(arrb); //必须先排序
        int index = Arrays.binarySearch(arrb, 20);
        System.out.println(index);
        int index2 = Arrays.binarySearch(arrb, 1, 3, 20);
        System.out.println(index + " " + index2);
    }
}

第五部分“类和对象”是Java区别于C等面向过程语言的重要部分，要理解“类”和“对象”的含义，在基础篇中学会如何定义类和创建使用对象即可，继承，多态，接口那些更精华的东西我们放在“核心篇”中归纳。

package com.kplayer

import java.util.Arrays;
import java.util.Date;

/*类和对象*/
public class KPStudent { //类权限约束成员权限，Default为包存取
    /*成员变量*/
    public String id;         //本类/同包类或子类/其它包类或子类
    protected String name;  //本类/同包类或子类
    private String sex;     //本类
    
    /*成员方法*/
    public String getId(){
        return this.id; //this代表本类对象的引用
    }
    
    private void setId(String id){
        this.id = id; //this表明第一个id是成员变量，区别于第二个id是形参
    }
    
    public KPStudent getStudent(){
        return this;
    }

    /*静态static
     *提供共享数据库或方法*/
    static final String SCHOOL = "LYEZ"; //静态常量
    static int age; //静态变量
    public static int checkAge(int age){ //静态方法，不可用this，不可直接调用非静态方法
        if(age > 30 || age < 10){
            System.out.println("Too Old/Young");
            return -1;
        }
        return 0;
    }
    static{
        System.out.println("Initialize..."); //类的初始化动作
    }
    
    /*构造方法
     *实例化对象时自动调用
     *未定义时自动创建一个无参构造方法
     *若定义的构造方法均有参，则不自动创建无参构造方法*/
    public KPStudent(){
        System.out.println("More Info is Better!");
    }

    public KPStudent(String id, String name){
        this.id = id;
        this.name = name;
        System.out.println("Create Stu Success!");
    }
    
    /*主方法*/
    public static void main(String[] args){
        KPStudent KPlayer = new KPStudent("1","KPlayer"); //创建对象    
        KPlayer.sex = "M"; //类成员变量赋值
        System.out.println(KPlayer.id + " " + KPlayer.name + " " + KPlayer.sex);
        
        KPStudent KPlayer2 = new KPStudent(); //创建对象
        KPlayer2.setId("2"); //调用方法
        KPlayer2.name = "KPlayer2";
        KPlayer2.sex = "M";
        KPlayer2.age = 45; //static连带修改KPlayer对象的age，用KPStudent.age更恰当
        System.out.println(KPlayer2.age + " " + KPlayer.age); //45 45
        
        //对象比较
        String s1 = new String("abc");
        String s2 = new String("abc");
        String s3 = s1;        
        System.out.println(s2 == s3); //false
        System.out.println(s2.equals(s3)); //true
    }
}

注：部分代码来源于《Java从入门到精通（第3版）》——清华大学出版社