2021-1-25
1. Basic use of Api (Api introduction)
2. Use Math tool class (Math common method)
Static method, can be called directly
3. System use (exit/currentTimeMillis)
system.exit When the code is executed to this method, it means that the virtual machine has stopped, and the status can usually be filled with 0
4. Object class (to String/equals and other methods)
5.Objects类(to String/isNull/nonNull)
6.BigDecimal class
BigDecimal construction: (understand)
BigDecimal common methods
7. Basic types of packaging (mainly master Integer)
8. Automatic boxing and automatic unboxing
9. Type conversion (conversion between int and String)
package com.wc.show;
/**
* @author wc
* @Date: 2021/01/26/19:55
*/
public class StudentDemo {
public static void main(String[] args) {
Student student=new Student();
student.setName("张三");
student.setId(""+1);
student.setId(String.valueOf(1));
System.out.println(student);
}
}
Case study
package com.wc.show;
/**
* @author wc
* @Date: 2021/01/26/20:09
*/
public class ArrayToInt {
public static void main(String[] args) {
String arr="99 23 34 12 15";
String[] s = arr.split(" ");
int[] numArr=new int[s.length];
for (int i = 0; i < s.length; i++) {
int parseInt = Integer.parseInt(s[i]);
numArr[i]=parseInt;
}
for (int i = 0; i < numArr.length; i++) {
System.out.println(numArr[i]);
}
}
}
10. Advanced operations of arrays (dichotomy)
package com.wc.show;
/**
* @author wc
* @Date: 2021/01/27/11:51
*/
public class ArrayMid {
public static void main(String[] args) {
int[] arr = {
1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
int number = 3;
int index = getIndex(arr, number);
System.out.println(index);
}
private static int getIndex(int[] arr, int number) {
//1.定义查找范围
int min = 0;
int max = arr.length - 1;
//循环查找min <= max
while (min <= max) {
//求出中间元素
int mid = (min + max) >> 1;
//判断number与mid的大小关系
if (arr[mid] > number) {
//要查找的元素在左边
max = mid - 1;
} else if (arr[mid] < number) {
//要查找的元素在右边
min = mid + 1;
}
if (arr[mid] == number) {
return mid;
}
}
//当min>max时，说明元素不在范围内
return -1;
}
}
11. Bubble sort
package com.wc.show;
/**
* @author wc
* @Date: 2021/01/27/15:31
*/
public class BubbleSort {
public static void main(String[] args) {
int[] arr = {
3, 5, 2, 1, 4};
//i < arr.length-1 减一是为了防止索引越界
for (int i = 0; i < arr.length - 1; i++) {
if (arr[i] > arr[i + 1]) {
//数据交换
int temp = arr[i];
arr[i] = arr[i + 1];
arr[i + 1] = temp;
}
}
printArr(arr);
for (int i = 0; i < arr.length - 1 - 1; i++) {
if (arr[i] > arr[i + 1]) {
//数据交换
int temp = arr[i];
arr[i] = arr[i + 1];
arr[i + 1] = temp;
}
}
printArr(arr);
for (int i = 0; i < arr.length - 1 - 1 - 1; i++) {
if (arr[i] > arr[i + 1]) {
//数据交换
int temp = arr[i];
arr[i] = arr[i + 1];
arr[i + 1] = temp;
}
}
printArr(arr);
for (int i = 0; i < arr.length - 1 - 1 - 1 - 1; i++) {
if (arr[i] > arr[i + 1]) {
//数据交换
int temp = arr[i];
arr[i] = arr[i + 1];
arr[i + 1] = temp;
}
}
printArr(arr);
}
private static void printArr(int[] arr) {
for (int i = 0; i < arr.length; i++) {
System.out.print(arr[i] + " ");
}
System.out.println();
}
}
Code optimization, common method for extraction
package com.wc.show;
/**
* @author wc
* @Date: 2021/01/27/15:31
*/
public class BubbleSort {
public static void main(String[] args) {
int[] arr = {
3, 5, 2, 1, 4};
changeArr(arr);
}
private static void changeArr(int[] arr) {
//第一层for是循环次数
for (int i = 0; i < arr.length - 1; i++) {
//第二层是交换次数
//i < arr.length-1 减一是为了防止索引越界
for (int j = 0; j < arr.length - 1 - i; j++) {
if (arr[j] > arr[j + 1]) {
//数据交换
int temp = arr[j];
arr[j] = arr[j + 1];
arr[j + 1] = temp;
}
}
}
printArr(arr);
}
private static void printArr(int[] arr) {
for (int i = 0; i < arr.length; i++) {
System.out.print(arr[i] + " ");
}
System.out.println();
}
}