Java Array Methods - Sådan udskrives en array i Java

Et array er en datastruktur, der bruges til at gemme data af samme type. Arrays gemmer deres elementer i sammenhængende hukommelsessteder.

I Java er arrays objekter. Alle metoder til klasseobjekt kan påberåbes i en matrix. Vi kan gemme et fast antal elementer i en matrix.

Lad os erklære en simpel primitiv type array:

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

Lad os nu prøve at udskrive det med System.out.println()metoden:

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

Hvorfor udskrev Java ikke vores array? Hvad sker der under emhætten?

Den System.out.println()metode konverterer objektet passerede vi ind i en snor ved at kalde String.valueOf(). Hvis vi ser på String.valueOf()metodens implementering, ser vi dette:

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

Hvis det indsendte objekt er null, returnerer det null, ellers kalder det obj.toString(). Til sidst System.out.println()kaldes for toString()at udskrive output.

Hvis objektets klasse ikke tilsidesætter Object.toString()implementeringen, kalder den Object.toString()metoden.

Object.toString()vender tilbage . I enkle vendinger returnerer det: "klasse navn @ objektets hash-kode".getClass().getName()+‘@’+Integer.toHexString(hashCode())

I vores tidligere output [[email protected]angiver [staterne, at dette er et array og Istår for int (typen af ​​array). 74a14482er den usignerede hexadecimale repræsentation af matrixens hash-kode.

Når vi opretter vores egne brugerdefinerede klasser, er det en bedste praksis at tilsidesætte Object.toString()metoden.

Vi kan ikke udskrive arrays i Java ved hjælp af en almindelig System.out.println()metode. I stedet for er det følgende måder, vi kan udskrive en matrix på:

  1. Sløjfer: til sløjfe og for hver sløjfe
  2. Arrays.toString() metode
  3. Arrays.deepToString() metode
  4. Arrays.asList() metode
  5. Java Iterator-grænseflade
  6. Java Stream API

Lad os se dem en efter en.

1. Sløjfer: til sløjfe og for hver sløjfe

Her er et eksempel på en for loop:

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!