Читать книгу OCP Oracle Certified Professional Java SE 17 Developer Study Guide - Jeanne Boyarsky - Страница 206

Before we move into branching statements, we need to introduce the concept of nested loops. A nested loop is a loop that contains another loop, including while, do/while, for, and for-each loops. For example, consider the following code that iterates over a two-dimensional array, which is an array that contains other arrays as its members. We cover multidimensional arrays in detail in Chapter 4, “Core APIs,” but for now, assume the following is how you would declare a two-dimensional array:

int[][] myComplexArray = {{5,2,1,3},{3,9,8,9},{5,7,12,7}}; for(int[] mySimpleArray : myComplexArray) { for(int i=0; i<mySimpleArray.length; i++) { System.out.print(mySimpleArray[i]+"\t"); } System.out.println(); }

Notice that we intentionally mix a for loop and a for-each loop in this example. The outer loop will execute a total of three times. Each time the outer loop executes, the inner loop is executed four times. When we execute this code, we see the following output:

5 2 1 3 3 9 8 9 5 7 12 7

Nested loops can include while and do/while, as shown in this example. See whether you can determine what this code will output:

int hungryHippopotamus = 8; while(hungryHippopotamus>0) { do { hungryHippopotamus -= 2; } while (hungryHippopotamus>5); hungryHippopotamus--; System.out.print(hungryHippopotamus+", "); }

The first time this loop executes, the inner loop repeats until the value of hungryHippopotamus is 4. The value will then be decremented to 3, and that will be the output at the end of the first iteration of the outer loop.

On the second iteration of the outer loop, the inner do/while will be executed once, even though hungryHippopotamus is already not greater than 5. As you may recall, do/while statements always execute the body at least once. This will reduce the value to 1, which will be further lowered by the decrement operator in the outer loop to 0. Once the value reaches 0, the outer loop will terminate. The result is that the code will output the following:

3, 0,

The examples in the rest of this section include many nested loops. You will also encounter nested loops on the exam, so the more practice you have with them, the more prepared you will be.