How to initialize an array in Java?

Initialization styles and common trip-ups

1. Declaration and Initialization style: Empty Arrays

To declare an array which will be filled later: String[] days = new String[7];

Trip-up #1: Arriving at the Wrong Station, Index out of bounds

Avoid:

int[] numbers = new int[3];
numbers[3] = 4; // Oops! This train doesn't go to station 3.
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Correct:

int[] numbers = new int[3];
numbers[2] = 4; // Yes Captain! Last Index in a size 3 array is 2.
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2. Declaration and Initialization style: Pre-filled Arrays

If you already know the rail stops (array elements): double[] temperatures = {98.6, 99.5, 97.1};

Trip-up #2: Boarding the Wrong train, Type mismatch

Avoid:

int[] primeNumbers = {2, 3.7, 5, 7}; // Oops! train 3.7 doesn't stop at "int" stations.
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Correct:

int[] primeNumbers = {2, 3, 5, 7}; // All aboard! All elements are "int" trains.
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Advanced conventions of array initialization

Array Initialization through looping

char[] grades = new char[5];
for (int i = 0; i < grades.length; i++) {
    grades[i] = getGrade(i); // getGrade(): The grade vending machine
}
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Array duplication and operations

Java's System.arraycopy method takes the wheel when you want to copy an existing array into a new one.

System.arraycopy(sourceTrain, 0, destinationTrain, 0, sourceTrain.length); // All aboard, please mind the gap!
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Multidimensional arrays

Initialize at the time of declaration:

int[][] 2DMatrix = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};
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Or use loops for dynamic assignments:

int[][] 2DMatrix = new int[3][3];
for (int i = 0; i < 2DMatrix.length; i++) {
    for (int j = 0; j < 2DMatrix[i].length; j++) {
        2DMatrix[i][j] = i * j; // Just some random multiplication for demo.
    }
}
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Wise usage of Java 8 and above features (the fancy city trains!)

Java 8 introduced streams, which can also be used for array initialization:

String[] stringArray = Stream.of("a", "b", "c", "d").toArray(String[]::new);
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Visualization

Think of array initialization in Java as setting up a bookshelf for your books:

Empty Bookshelf (📚): [ , , , ]

Initialization with specific size but no books yet:

Book[] bookShelf = new Book[4]; // A bookshelf with space for 4 books
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Empty Bookshelf (📚): [_, _, _, _]

Filling in the books (array elements):

bookShelf[0] = new Book("Java Fundamentals");
bookShelf[1] = new Book("Effective Java");
bookShelf[2] = new Book("Spring in Action");
bookShelf[3] = new Book("Clean Code");
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Filled Bookshelf (📚): [📘, 📗, 📙, 📕]

This is exactly like organizing your precious book collection on a shelf!