„Java“ masyvo metodai - kaip išspausdinti masyvą „Java“

Masyvas yra duomenų struktūra, naudojama to paties tipo duomenims saugoti. Masyvai saugo savo elementus gretimose atminties vietose.

„Java“ masyvai yra objektai. Visus klasės objekto metodus galima naudoti masyve. Masyve galime laikyti fiksuotą elementų skaičių.

Paskelbkime paprastą primityvų masyvo tipą:

int[] intArray = {2,5,46,12,34};

Dabar pabandykime atsispausdinti tokiu System.out.println()metodu:

System.out.println(intArray); // output: [[email protected]

Kodėl „Java“ nespausdino mūsų masyvo? Kas vyksta po gaubtu?

System.out.println()Būdas paverčia objektą mes perėjo į eilutę paskambinus String.valueOf(). Jei pažvelgsime į String.valueOf()metodo įgyvendinimą, pamatysime tai:

public static String valueOf(Object obj) { return (obj == null) ? "null" : obj.toString(); }

Jei perduotas objektas nullgrąžina nulį, kitu atveju jis iškviečia obj.toString(). Galų gale, System.out.println()skambučiai toString()spausdinti išvestį.

Jei to objekto klasė nepaiso Object.toString()įdiegimo, jis iškvies Object.toString()metodą.

Object.toString()grįžta . Paprasčiau tariant, jis pateikia: „klasės pavadinimas @ objekto maišos kodas“.getClass().getName()+‘@’+Integer.toHexString(hashCode())

Mūsų ankstesnės produkcijos [[email protected], The [teigia, kad tai yra masyvas ir Istendai INT (masyvo tipo). 74a14482yra nepasirašytas šešioliktainis masyvo maišos kodo atvaizdavimas.

Kai mes kuriame savo pačių pritaikytas klases, geriausia praktika nepaisyti Object.toString()metodo.

Negalime spausdinti masyvų „Java“ naudodami paprastą System.out.println()metodą. Tai yra šie masyvo spausdinimo būdai:

  1. Kilpos: kilpai ir kiekvienai kilpai
  2. Arrays.toString() metodas
  3. Arrays.deepToString() metodas
  4. Arrays.asList() metodas
  5. „Java Iterator“ sąsaja
  6. „Java Stream“ API

Pažiūrėkime juos po vieną.

1. Kilpos: kilpai ir kiekvienai kilpai

Štai „for loop“ pavyzdys:

int[] intArray = {2,5,46,12,34}; for(int i=0; i
    

All wrapper classes override Object.toString() and return a string representation of their value.

And here's a for-each loop:

int[] intArray = {2,5,46,12,34}; for(int i: intArray){ System.out.print(i); // output: 25461234 }

2. Arrays.toString() method

Arrays.toString() is a static method of the array class which belongs to the java.util package. It returns a string representation of the contents of the specified array. We can print one-dimensional arrays using this method.

Array elements are converted to strings using the String.valueOf() method, like this:

int[] intArray = {2,5,46,12,34}; System.out.println(Arrays.toString(intArray)); // output: [2, 5, 46, 12, 34]

For a reference type of array, we have to make sure that the reference type class overrides the Object.toString() method.

For example:

public class Test { public static void main(String[] args) { Student[] students = {new Student("John"), new Student("Doe")}; System.out.println(Arrays.toString(students)); // output: [Student{name='John'}, Student{name='Doe'}] } } class Student { private String name; public Student(String name){ this.name = name; } public String getName() { return name; } public void setName(String name) { this.name = name; } @Override public String toString() { return "Student{" + "name='" + name + '\'' + '}'; } }

This method is not appropriate for multidimensional arrays. It converts multidimensional arrays to strings using Object.toString() which describes their identities rather than their contents.

For example:

// creating multidimensional array int[][] multiDimensionalArr = { {2,3}, {5,9} }; System.out.println(Arrays.toString(multiDimensionalArr)); // output: [[[email protected], [[email protected]]

With the help of Arrays.deepToString(), we can print multidimensional arrays.

3. Arrays.deepToString() method

Arrays.deepToString() returns a string representation of the “deep contents” of the specified array.

If an element is an array of primitive type, it is converted to a string by invoking the appropriate overloading of Arrays.toString() .

Here is an example of the primitive type of multidimensional array:

// creating multidimensional array int[][] multiDimensionalArr = { {2,3}, {5,9} }; System.out.println(Arrays.deepToString(multiDimensionalArr)); // output: [[2, 3], [5, 9]]

If an element is an array of reference type, it is converted to a string by invoking Arrays.deepToString() recursively.

Teacher[][] teachers = {{ new Teacher("John"), new Teacher("David") }, {new Teacher("Mary")} }; System.out.println(Arrays.deepToString(teachers)); // output: [[Teacher{name='John'}, Teacher{name='David'}],[Teacher{name='Mary'}]]

We have to override Object.toString() in our Teacher class.

If you are curious as to how it does recursion, here is the source code for the Arrays.deepToString() method.

NOTE: Reference type one-dimensional arrays can also be printed using this method. For example:

Integer[] oneDimensionalArr = {1,4,7}; System.out.println(Arrays.deepToString(oneDimensionalArr)); // output: [1, 4, 7]

4. Arrays.asList() method

This method returns a fixed-size list backed by the specified array.

Integer[] intArray = {2,5,46,12,34}; System.out.println(Arrays.asList(intArray)); // output: [2, 5, 46, 12, 34]

We have changed the type to Integer from int, because List is a collection that holds a list of objects. When we are converting an array to a list it should be an array of reference type.

Java calls Arrays.asList(intArray).toString() . This technique internally uses the toString() method of the type of the elements within the list.

Another example with our custom Teacher class:

Teacher[] teacher = { new Teacher("John"), new Teacher("Mary") }; System.out.println(Arrays.asList(teacher)); // output: [Teacher{name='John'}, Teacher{name='Mary'}]

NOTE: We can not print multi-dimensional arrays using this method. For example:

Teacher[][] teachers = {{ new Teacher("John"), new Teacher("David") }, { new Teacher("Mary") }}; System.out.println(Arrays.asList(teachers)); // output: [[Lcom.thano.article.printarray.Teacher;@1540e19d, [Lcom.thano.article.printarray.Teacher;@677327b6]

5. Java Iterator Interface

Similar to a for-each loop, we can use the Iterator interface to loop through array elements and print them.

Iterator object can be created by invoking the iterator() method on a Collection. That object will be used to iterate over that Collection’s elements.

Here is an example of how we can print an array using the Iterator interface:

Integer[] intArray = {2,5,46,12,34}; // creating a List of Integer List list = Arrays.asList(intArray); // creating an iterator of Integer List Iterator it = list.iterator(); // if List has elements to be iterated while(it.hasNext()) { System.out.print(it.next()); // output: 25461234 }

6. Java Stream API

The Stream API is used to process collections of objects. A stream is a sequence of objects. Streams don’t change the original data structure, they only provide the result as per the requested operations.

With the help of the forEach() terminal operation we can iterate through every element of the stream.

For example:

Integer[] intArray = {2,5,46,12,34}; Arrays.stream(intArray).forEach(System.out::print); // output: 25461234

Now we know how to print an array in Java.

Thank you for reading.

Cover image by Aziz Acharki on Unsplash.

You can read my other articles on Medium.

Happy Coding!