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This article mainly introduces "what are the knowledge points of the Java array". In the daily operation, I believe that many people have doubts about the knowledge points of the Java array. The editor consulted all kinds of materials and sorted out simple and easy-to-use operation methods. I hope it will be helpful to answer the questions of "what are the knowledge points of the Java array?" Next, please follow the editor to study!

The definition of array 1. Overview

If there is a class whose grades need to be stored, what method should be used?

As we learned before, you can define multiple variables to store different grades. But if there are more than 1000 students, is it possible to define more than 1000 variables? Of course not, we need to use our array.

two。 Static initialization array

Features: when defining an array, assign values to the array directly, and the length of the array is determined by the system.

Common format:

Data type [] array name = {element 1, element 2, element 3, … }; for example: int [] array= {1 array= 2, 3, 4, 5}; double [] scores = {88. 5,99.5, 59.5}; 3. Dynamically initialize array

Features: when defining an array, determine the type of the element and the length of the array, and then store the data

Common format:

Data type [] array name = new data type [length]; for example: int [] array= new int [5]; double [] scores = new double [3]

Default value:

Data type specific definition type default value basic type

Byte 、 short 、 char 、 int 、 long

0

Float 、 double

0.0

Boolean

False reference type

Classes, interfaces, arrays, String

Null4. Summary

Arrays are suitable for large amounts of data of the same type

Static initialization is suitable for knowing element values

Dynamic initialization is suitable for not knowing which data to store.

The attribute of the array 1. Visit

The general way to access an array is:

Array name [index]

Example:

/ / statically initialize the array int [] array= {1 array [0]); / / output 1System.out.println (array [1]); / / output 2System.out.println (array [3]); / / output 42. Length

Length you can call length directly to get the length of the array.

Example:

/ / statically initialize the array int [] array= {1 _ array.length _ 3 _ 4 _ 5}; System.out.println (array.length); / / call the method with the output length of 5pm / maximum index array.length-13. Ergodic

Traversal is an array element access, mainly used in search, data statistics.

We learned day 7: loop structure, day 6: branch structure, and then iterate through an array through the for loop

Example:

Given the element {10pc8, 9, 4, 5, 6, 71, 2, 3, 9, 99}, use a static array to store and output elements greater than 5 in the array?

Coding implementation:

/ / statically initialize the array int [] array= {10, 8, 9, 4, 5, 5, 6, 8, 71, 2, 3, 9, 99}; for (int item0, System.out.println, i5) System.out.println (array [I]);}

Output result:

10 8 9 6 8 71 9 99 memory diagram

When the program is running, Java needs to allocate space in memory and divides the space into different regions.

Stack memory: stores local variables and disappears immediately after use

Heap memory: stores the contents (objects, entities) out of new. The address is reclaimed when the garbage collector is idle.

1. Single array memory graph

The following code to create an array implements its memory diagram

Coding implementation:

/ / dynamically initialize the array int [] arr=new int [3]; System.out.println (arr); System.out.println (arr [0]); System.out.println (arr [1]); System.out.println (arr [2]); / / modify the value arr [0] = 100 arr [2] = 200 arr System.out.println (arr); System.out.println (arr [0]); System.out.println (arr [1]); System.out.println (arr [2])

Output result:

[I@15db9742000 [I@15db97421000200

Explain the principle:

Dynamic initialization first generates a new and an arr address value in heap memory, depending on the result of the compiler, assume 001. Due to dynamic initialization, each element has an initial value, as shown in the table above. We output the element, first access the array name and address, to the heap memory subscript, and then output the element value.

The process of modifying array values is the same as viewing, but with one more step of modification, as shown in the following figure:

two。 Most group memory graph

Multiple arrays and single arrays use the same principle of memory, so I won't talk too much about it here.

3. The array points to the same memory

If we change the address value of the two arrays to the same, what the modified result should be, as in the following code.

Coding implementation:

/ dynamically initialize the array int [] arr=new int [3]; arr [0] = 100 System.out.println arr [1] = 200 arr arr [2] = 300 System.out.println system. Out.println (arr); System.out.println (arr [0]); System.out.println (arr [1]); System.out.println (arr [2]); int [] arr2=arr;arr2 [0] = 111 System.out.println Arr2 [1] = 222 transarr2 [2] = 333 boot System.out.println (arr); System.out.println (arr [0]) System.out.println (arr2); System.out.println (arr2 [0])

Output result:

[I @ 15db9742100200300 [I @ 15db9742111 [I @ 15db9742111]

Explain the principle:

The address of the first array in heap memory is 001, and the second array is also 001, so modifying the value of the second array is actually the same array memory. The value of the first array also changes, and the result is as follows:

FAQ 1. Index out-of-bounds / / static initialization array int [] array= {1, array 2, 3}; index (array [3])

After the above code runs, the following error exception occurs:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 3

Explanation: we static the initial array to give 3 numbers, the maximum index is 2, when we access 3, we will report an error.

two。 Null pointer exception / / dynamically initialize array int [] array= new int [3]; array=null;System.out.println (array [0])

After the above code runs, the following error exception occurs:

Exception in thread "main" java.lang.NullPointerException

Explanation: we set the array to null so that the accessed array does not point to the data in the heap memory

At this point, the study of "what are the knowledge points of Java array" is over. I hope to be able to solve your doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

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