Java-Array-Methoden - So drucken Sie ein Array in Java

Ein Array ist eine Datenstruktur, in der Daten desselben Typs gespeichert werden. Arrays speichern ihre Elemente an zusammenhängenden Speicherorten.

In Java sind Arrays Objekte. Alle Methoden des Klassenobjekts können in einem Array aufgerufen werden. Wir können eine feste Anzahl von Elementen in einem Array speichern.

Deklarieren wir einen einfachen primitiven Array-Typ:

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

Versuchen wir nun, es mit der folgenden System.out.println()Methode zu drucken :

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

Warum hat Java unser Array nicht gedruckt? Was passiert unter der Haube?

Die System.out.println()Methode konvertiert das übergebene Objekt durch Aufrufen in einen String String.valueOf(). Wenn wir uns die String.valueOf()Implementierung der Methode ansehen , sehen wir Folgendes:

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

Wenn das übergebene Objekt nullnull ist, wird es zurückgegeben, andernfalls wird es aufgerufen obj.toString(). Schließlich System.out.println()ruft toString()die Ausgabe zu drucken.

Wenn die Klasse dieses Objekts die Object.toString()Implementierung nicht überschreibt , wird die Object.toString()Methode aufgerufen .

Object.toString()kehrt zurück . In einfachen Worten wird Folgendes zurückgegeben: "Klassenname @ Objekt-Hash-Code".getClass().getName()+‘@’+Integer.toHexString(hashCode())

In unserer vorherigen Ausgabe [[email protected]gibt das an [, dass dies ein Array ist, und Isteht für int (den Typ des Arrays). 74a14482ist die vorzeichenlose hexadezimale Darstellung des Hash-Codes des Arrays.

Wenn wir eigene benutzerdefinierte Klassen erstellen, empfiehlt es sich, die Object.toString()Methode zu überschreiben .

Wir können Arrays in Java nicht mit einer einfachen System.out.println()Methode drucken . Stattdessen können wir auf folgende Weise ein Array drucken:

  1. Schleifen: für Schleife und für jede Schleife
  2. Arrays.toString() Methode
  3. Arrays.deepToString() Methode
  4. Arrays.asList() Methode
  5. Java Iterator-Schnittstelle
  6. Java Stream API

Lassen Sie uns sie eins nach dem anderen sehen.

1. Schleifen: für Schleife und für jede Schleife

Hier ist ein Beispiel für eine for-Schleife:

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!

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