第四知识单元

工具类与算法

Java程序设计 第7讲，主讲人：李欣

Created: 2023-12-12 Tue 21:30

0.1. 互动课堂

Click to host the seminar.

0.2. 签到

https://xin.blue/tool/attendance/

0.3. 本次课的目标

  • 基础类库
    • 了解基础类库的层次结构；
    • 理解和掌握基础类库的使用方法；
    • 熟练使用 Math System Calendar 类及基本数据类型的包装类。
  • 向量
    • 对比数组，理解并掌握向量类的优点，并熟练使用。
  • 字符串
    • 理解并掌握 String 类及相关操作；
    • 了解 StringBuffer 类的使用。

1. 基础类库

1.1. Object类

Object类 是Java程序中 所有类 的 直接 或 间接 父类 ，也是类库中 所有类 的 父类 。 所有其他类都是从 Object类 派生出来的。

https://docs.oracle.com/en/java/javase/18/docs/api/java.base/java/lang/Object.html

1.1.1. protected Object clone()

class Main {
    public static void main(String[] args) throws CloneNotSupportedException {
        TestObject obj1 = new TestObject();
        TestObject obj2 = (TestObject)obj1.clone();
        System.out.println("Hash code of obj1: " + obj1.hashCode() + "\n" +
                           "Hash code of obj2: " + obj2.hashCode());
    }
}
class TestObject implements Cloneable {
    public Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}

Hash code of obj1: 1878246837
Hash code of obj2: 929338653

1.1.2. public boolean equals(Object obj)

class Main {
    public static void main(String[] args) throws CloneNotSupportedException {
        TestObject obj1 = new TestObject();
        TestObject obj2 = (TestObject)obj1.clone();
        System.out.println(obj1.equals(obj2) + ", " + obj1.test + ", " + obj2.test);
    }
}
class TestObject implements Cloneable {
    public int test = 1;
    public Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}

false, 1, 1

1.1.3. public final Class<?> getClass()

class Main {
    public static void main(String[] args) throws CloneNotSupportedException {
        TestObject obj1 = new TestObject();
        Object obj2 = obj1.clone();
        Object obj3 = new Object();
        System.out.println(obj1.getClass() + "\n" + obj2.getClass() + "\n" + obj3.getClass());
    }
}
class TestObject implements Cloneable {
    public int test = 1;
    public Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}

class TestObject
class TestObject
class java.lang.Object

public class Main {
    public static void main(String[] args) {
        Car<Integer> iCar = new Car<Integer>();
        Car<String> strCar = new Car<String>();
        iCar.set(3);
        strCar.set("three");
        System.out.println(iCar.get() + ", " + strCar.get());
    }
}
class Car<T> { // T stands for "Type"
    private T t;
    public void set(T t) { this.t = t; }
    public T get() { return t; }
}

3, three

1.1.4. public String toString()

public class Main {
    public static void main(String[] args) {
        Car<Integer> iCar = new Car<Integer>();
        Car<String> strCar = new Car<String>();
        iCar.set(3);
        strCar.set("three");
        System.out.println(iCar.toString() + ", " + strCar.toString());
    }
}
class Car<T> { // T stands for "Type"
    private T t;
    public void set(T t) { this.t = t; }
    public T get() { return t; }
}

Car@6ff3c5b5, Car@3764951d

public class Main {
    public static void main(String[] args) {
        Car<Integer> iCar = new Car<Integer>();
        Car<String> strCar = new Car<String>();
        iCar.set(3);
        strCar.set("three");
        System.out.println(iCar.toString() + ", " + strCar.toString());
    }
}
class Car<T> { // T stands for "Type"
    private T t;
    public void set(T t) { this.t = t; }
    public T get() { return t; }
    public String toString() {
        return "Car: " + get();
    }
}

Car: 3, Car: three

1.2. 数据类型类

数据类型类 基本数据类型
Boolean boolean
Character char
Double double
Float float
Integer int
Long long 
System.out.println(Integer.MAX_VALUE + ", " +
                   Integer.MIN_VALUE + ", " +
                   Integer.TYPE + ", " +
                   Integer.BYTES + ", " +
                   Integer.SIZE);

2147483647, -2147483648, int, 4, 32

Integer _i = new Integer("1");
double d = _i.doubleValue();
long l = _i.longValue();
int i = _i.intValue();
System.out.println(d + ", " + l + ", " + i);

1.0, 1, 1

System.out.println(Integer.parseInt("123") + 123);
System.out.println((new Integer(123).toString()) + 123);
System.out.println(Integer.valueOf("123").intValue() + 123);
System.out.println(Float.valueOf("123").floatValue() + 123);

246
123123
246
246.0

1.3. Math类

System.out.println(Math.E);
System.out.println(Math.PI);
System.out.print(Math.IEEEremainder(28, 5) + ", ");
System.out.println(28 % 5);
System.out.print(Math.IEEEremainder(-16.3, 4.1) + ", ");
System.out.println(-16.3 % 4.1);

2.718281828459045
3.141592653589793
-2.0, 3
0.09999999999999787, -4.000000000000002
  IEEERemainder %
3 / 2 -1 1
4 / 2 0 0
10 / 3 1 1
11 / 3 -1 2
27 / 4 -1 3
28 / 5 -2 3
17.8 / 4 1.8 1.8
17.8 / 4.1 1.4 1.4
-16.3 / 4.1 0.0999999999999979 -4
17.8 / -4.1 1.4 1.4
-17.8 / -4.1 -1.4 -1.4

1.4. System类

System.out.println(System.currentTimeMillis());
System.out.println(new java.util.Date(System.currentTimeMillis()));

1664297395479
Wed Sep 28 00:49:55 CST 2022

2. 向量

https://docs.oracle.com/en/java/javase/18/docs/api/java.base/java/util/Vector.html

2.1. 添加元素

import java.util.*;
class Main {
    public static void main(String[] args) {
        Vector v1 = new Vector(); // Case 1: Creating a default vector
        v1.add(1);
        v1.add(2);
        v1.add("Hello");
        v1.add("Java");
        v1.add(3);
        System.out.println("Vector v1 is " + v1); // Printing the vector elements to the console
        Vector<Integer> v2 = new Vector<Integer>(); // Case 2: Creating generic vector
        v2.add(1);
        v2.add(2);
        v2.add(3);
        v2.add(1, 0);
        System.out.println("Vector v2 is " + v2); // Printing the vector elements to the console
    }
}

Vector v1 is [1, 2, Hello, Java, 3]
Vector v2 is [1, 0, 2, 3]

2.2. 删除元素

import java.util.*;
class Main {
    public static void main(String[] args) {
        int n = 5;
        Vector<Integer> v = new Vector<Integer>(n);
        for (int i = 1; i <= n; i++)
            v.add(i); // Appending new elements at the end of the vector
        System.out.print(v + ", "); // Printing elements
        v.remove(3); // Remove element at index 3
        System.out.print(v + ", "); // Displaying the vector after deletion
        v.add(3);
        System.out.print(v + ", "); // Displaying the vector after deletion
        v.remove(new Integer(3));
        System.out.println(v); // Displaying the vector after deletion
        for (int i = 0; i < v.size(); i++) // iterating over vector elements using for loop
            System.out.print(v.get(i) + " "); // Printing elements one by one
        System.out.println();
        v.removeAllElements();
        System.out.println(v); // Displaying the vector after deletion
    }
}

[1, 2, 3, 4, 5], [1, 2, 3, 5], [1, 2, 3, 5, 3], [1, 2, 5, 3]
1 2 5 3 
[]

2.3. 修改元素

import java.util.*;
public class Main {
    public static void main(String[] args) {
        Vector<Integer> vector = new Vector<Integer>(); // Creating an empty Vector
        vector.add(12);
        vector.add(23);
        vector.add(22);
        vector.add(10);
        vector.add(20);
        System.out.println("Vector: " + vector); // Displaying the Vector
        System.out.println("The Object that is replaced is: " +
                           vector.set(0, 21)); // Using set() method to replace 12 with 21
        System.out.println("The Object that is replaced is: " +
                           vector.set(4, 50)); // Using set() method to replace 20 with 50
        // Displaying the modified vector
        System.out.println("The new Vector is: " + vector);
    }
}

Vector: [12, 23, 22, 10, 20]
The Object that is replaced is: 12
The Object that is replaced is: 20
The new Vector is: [21, 23, 22, 10, 50]

2.4. 查找元素

import java.util.*;
public class Main {
    public static void main(String[] args) {
        Vector vector = new Vector(); // Creating an empty Vector
        vector.add(12);
        vector.add("Hello");
        vector.add("Java");
        vector.add(10);
        vector.add(20);
        vector.add("Welcome");
        vector.add("OK!");
        vector.add("Welcome");
        System.out.println(vector.contains("Java") + ", " +
                           vector.contains("World") + ", " +
                           vector.contains(20));
        if(vector.contains(20)) {
            System.out.println(vector.indexOf(20));
        }
        for(int i = 0; (i = vector.indexOf("Welcome", i)) != -1; i++)
            System.out.print(i + " ");
    }
}

true, false, true
4
5 7

3. 字符串

3.1. 字符串长度

public int length() 

String s = "Hello!";
System.out.println(s.length());
s = "你好！";
System.out.println(s.length());

6
3

3.2. 判断字符串是否为空

public boolean isEmpty() 

class Main {
    public static void main(String [] args) {
        System.out.println("".isEmpty());
        System.out.println(".".isEmpty());
    }
}

true
false

3.3. 判断字符串前缀和后缀

class Main {
    public static void main(String[] args) {
        String id1 = "11010720501111907X";
        String id2 = "110107205011116688";
        checkGender(id1);
        checkGender(id2);
    }
    public static void checkGender(String id) {
        String s = id.substring(0, 17);
        if(s.endsWith("0") || s.endsWith("2") || s.endsWith("4") || s.endsWith("6") || s.endsWith("8")) {
            System.out.println(s + " 性别：女");
        } else {
            System.out.println(s + " 性别：男");
        }
    }
}

11010720501111907 性别：男
11010720501111668 性别：女