Java SE 学习笔记--Lesson 04

 

Java SE Lesson 4

 

1. 包装类（Wrapper Class）。针对于原生数据类型的包装。所有的包装类（8个）都位于java.lang包下。Java中的8个包装类分别是：Byte, Short, Integer, Long, Float, Double, Character, Boolean。他们的使用方式都是一样的，可以实现原生数据类型与包装类型的双向转换。

 

public class IntegerTest

{

       public static void main(String[] args)

       {

              int a = 10;

 

              Integer integer = new Integer(a);

      

              int b = integer.intValue();

 

              System.out.println(a == b);

       }

}

 

 

2. 数组（Array）：相同类型数据的集合就叫做数组。

 

3. 如何定义数组。 type[]变量名 = new type[数组中元素的个数];可以按照下列方式定义长度为10的数组： int[] a = new int[10]; 或者 int a[] = new int[10];

 

4. 数组中的元素索引是从0开始的。对于数组来说，最大的索引==数组的长度– 1。

 

5. 定义数组的第3种方式： type[]变量名 = {new type[]}{逗号分隔的初始化值列表};

 

6. Java中的每个数组都有一个名为length的属性，表示数组的长度。length属性是public，final，int的。数组长度一旦确定，就不能改变大小。

 

public class ArrayTest

{

       public static void main(String[] args)

       {

              /*

              int[] a = new int[4];

 

              a[0] = 1;

              a[1] = 2;

              a[2] = 3;

              a[3] = 4;

 

              System.out.println(a[3]);

              */

 

              /*

              int a[] = new int[2];

 

              a[0] = 1;

              a[1] = 2;

 

              System.out.println(a[1]);

              */

 

              /*

              int[] a = {1, 2, 3, 4};

 

              System.out.println(a[2]);

 

              int[] b = new int[]{1, 2, 3, 4};

 

              System.out.println(b[3]);

              */

             

              /*

              int[] a = new int[100];

             

             for(int i = 0; i < a.length; i++)

             {

                    a[i] = i + 1;

             }

 

             for(int i = 0; i < a.length; i++)

             {

                    System.out.println(a[i]);

             }

 

              //a.length = 200;

              */

 

              /*

              int[] a = new int[4];

 

              System.out.println(a[0]);

 

              boolean[] b = new boolean[3];

 

              System.out.println(b[2]);

      

              */

 

              int[] a = {1, 2 ,3};

              int[] b = {1, 2, 3};

 

              System.out.println(a.equals(b));

 

 

       }

}

 

 

7.       int[] a = new int[10]，其中a是一个引用，它指向了生成的数组对象的首地址，数组中每个元素都是int类型，其中仅存放数据值本身。

 

 

 

 

 

 

 

public class ArrayTest2

{

       public static void main(String[] args)

       {

              Person[] p = new Person[3];

 

              p[0] = new Person(10);

              p[1] = new Person(20);

              p[2] = new Person(30);

 

              for(int i = 0; i < p.length; i++)

              {

                     System.out.println(p[i].age);

              }

 

              Person[] p2 = new Person[5];

 

              for(int i = 0; i < p2.length; i++)

              {

                     System.out.println(p2[i]);

              }

       }

}

 

 

class Person

{

       int age;

 

       public Person(int age)

       {

              this.age = age;

       }

}

 

 

 

 

 

public class ArrayTest3

{

       public static void main(String[] args)

       {

              Student[] s = new Student[100];

 

              for(int i = 0; i < s.length; i++)

              {

                     s[i] = new Student();

 

                     s[i].name = i % 2 == 0 ? "zhangsan" : "lisi";

              }

 

              for(int i = 0; i < s.length; i++)

              {

                     System.out.println(s[i].name);

              }

       }

}

 

class Student

{

       String name;

}

 

 

8. 二维数组。二维数组是一种平面的二维结构，本质上是数组的数组。二维数组的定义式：type[][] a = new type[2][3];

 

public class ArrayTest4

{

       public static void main(String[] args)

       {

              int[][] a = new int[2][3];

 

              //System.out.println(i[0] instanceof int[]);

             

              int m = 0;

 

              for(int i = 0; i < 2; i++)

              {

                     for(int j = 0; j < 3; j++)

                     {

                            m++;

                            a[i][j] = 2 * m;            

                     }

              }

             

       }

}

 

 

 

 

 

public class ArrayTest5

{

       public static void main(String[] args)

       {

              /*

              int[][] a = new int[3][];

 

              a[0] = new int[2];

              a[1] = new int[3];

              a[2] = new int[1];

              */

 

              //int[][] a = new int[][3];

             

              //int[] a = new int[]{1, 2 ,3};

 

              int[][] a = new int[][]{ {1 ,2 ,3}, {4}, {5, 6, 7, 8} };

 

              for(int i = 0; i < a.length; i++)

              {

                     for(int j = 0; j < a[i].length; j++)

                     {

                            System.out.print(a[i][j] + " ");

                     }

 

                     System.out.println();

              }

       }

}

 

 

9.三维数组。type[][][] a = new type[2][3][4];

 

public class ThreeDimensionArrayTest

{

       public static void main(String[] args)

       {

              int[][][] a = new int[2][3][4];

 

              System.out.println(a instanceof int[][][]);

              System.out.println(a[0] instanceof int[][]);

              System.out.println(a[0][0] instanceof int[]);

 

              for(int i = 0; i < a.length; i++)

              {

                     for(int j = 0; j < a[i].length; j++)

                     {

                            for(int k = 0; k < a[i][j].length; k++)

                            {

                                   a[i][j][k] = 100;

                            }

                     }

              }

       }

 

 

}

 

10.冒泡排序。（掌握交换排序，快速排序的原理与实现方式）

 

public class BubbleSortTest

{

       public static void bubbleSort(int[] a)

       {

              for(int i=0;i<a.length-1;i++)

              {

                     for(int j=0;j<a.length-1-i;j++)

                     {

                            if(a[j]>a[j+1])

                            {

                                   int temp = a[j];

                                   a[j] = a[j+1];

                                   a[j+1] = temp;

                            }

                     }

 

              }

 

 

              for(int k=0;k<a.length;k++)

              {

 

              System.out.println(a[k]);

             

              }

       }

 

public static void main(String[] args)

{

       int[] a = {4,6,3,8,7,9,2,1,0};

       bubbleSort(a);

 

}

}

 

 

 

11. 二分查找（Binary Search）：待查找的数组要有序。

 

public class ArraySearchTest

{

       public static int search(int[] array, int value)

       {

              for(int i = 0; i < array.length; i++)

              {

                     if(value == array[i])

                     {

                            return i;

                     }

              }

 

              return -1;

       }

 

 

       public static int binarySearch(int[] array, int value)

       {

              int low = 0;

              int high = array.length - 1;

              int middle;

 

              while(low <= high)

              {

                     middle = (low + high) / 2;

 

                     for(int i = 0; i < array.length; i++)

                     {

                            System.out.print(array[i]);

 

                            if(i == middle)

                            {

                                   System.out.print("#");

                            }

 

                            System.out.print(" ");

                     }

                    

                     System.out.println();

 

 

                     if(array[middle] == value)

                     {

                            return middle;

                     }

 

                     if(value < array[middle])

                     {

                            high = middle - 1;

                     }

                     if(value > array[middle])

                     {

                            low = middle + 1;

                     }

              }

 

              return -1;

 

       }

 

       public static void main(String[] args)

       {

              int[] a = new int[]{1, 2, 3, 4, 5, 6, 7, 8, 9};

 

              int value = 11;

 

              int index = search(a, value);

 

              System.out.println(index);

 

              System.out.println("-----------------");

 

              int[] b = new int[]{1, 2, 3, 4, 5, 6, 7, 8, 9};

 

              int index2 = binarySearch(b , 10);

 

              System.out.println(index2);

       }

}

 

 

12. 随机生成50个数字（整数），每个数字的范围是[10, 50]，统计每个数字出现的次数以及出现次数最多的数字与它的个数，最后将每个数字及其出现次数打印出来，如果某个数字出现次数为0，则不要打印它。打印时按照数字的升序排列。