for 循环（）简单到高阶

for循环的初始意义是遍历一串具有相同特性的值
1、遍历数组，根据索引去求值

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public class ForDemo1 {public static void main(String[] args) {int[] arr = {1,2,3,4,5,6,7};for (int i = 0; i < arr.length; i++) {System.out.println("arr["+i+"] = "+arr[i]);}}
}

for循环的嵌套（双重for循环）
遍历数组

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public class ForDemo2 {public static void main(String[] args) {int[] arr = {1,2,3,4};for (int i = 0; i < arr.length; i++) {for (int j = 0; j < arr.length; j++) {System.out.print("arr["+i+"] = "+arr[i]+"\t\t"); //i 不走内层循环所以一直不变System.out.println("arr["+j+"] = "+arr[j]);//    j走外层循环，值J++}System.out.println();//内层for循环结束了，保留一行}}
}

for循环的嵌套（双重for循环）,j从i开始
规律：跟自己及一会的数组进行比较，带最后一个

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public class ForDemo3 {public static void main(String[] args) {int[] arr = {1,2,3,4};for (int i = 0; i < arr.length; i++) {for (int j = i; j < arr.length ; j++) {     //外层i与内层i有关（次数保持一致）System.out.print( "i :" +"arr["+i+"] = "+arr[i]+"\t\t");System.out.println("j :"+"arr["+j+"] = "+arr[j]);}System.out.println();}}
}

for循环的嵌套（双重for循环）,j的长度限定为length - 1

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package com.shujia.demotest;public class ForDemo4 {public static void main(String[] args) {int[] arr = {1,2,3,4};for (int i = 0; i < arr.length; i++) {for (int j = 0; j < arr.length -  i ; j++) {     //外层i与内层i有关（次数保持一致）System.out.print( "i :" +"arr["+i+"] = "+arr[i]+"\t\t");System.out.println("j :"+"arr["+j+"] = "+arr[j]);}System.out.println();}}
}

每个数只跟自己后面的数进行比较

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public class ForDemo3 {public static void main(String[] args) {int[] arr = {1,2,3,4};for (int i = 0; i < arr.length; i++) {for (int j = i + 1; j < arr.length ; j++) {     //外层i与内层i有关（次数保持一致）System.out.print( "i :" +"arr["+i+"] = "+arr[i]+"\t\t");System.out.println("j :"+"arr["+j+"] = "+arr[j]);}System.out.println();}}
}