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These learning objects explain arrays in the Java program language. Arrays are a data structure for efficiently accessing a large number of components of the same type.

Learning objects for arrays

Concept An array is a sequence of elements of the same type; the type of the elements can be a primitive types such as int , or a predefined or user-defined class type. To access an element of an array, an index is given; this may be any expression of type int , including an integer literal or a variable.

The source code of these learning objects can be found in array.zip .

LO Topic Java Files (.java) Prerequisites
"Array objects" Array objects Array01A, B
"Array initializers" Array initializers Array02 1
"Passing arrays as parameters" Passing arrays as parameters Array03 2
"Returning an array from a method" Returning an array from a method Array04 2
"Array assignment can create garbage" Array assignment can create garbage Array05 4
"Two-dimensional arrays" Two-dimensional arrays Array06 3
"Arrays of arrays" Arrays of arrays Array07 6
"Ragged Arrays" Ragged arrays Array08 6
"Arrays of objects" Arrays of objects Array09 3

The example used in LO 1 through LO 4 is to fill an array with a sequence of fibonacci numbers (0,1,1,2,3,5,8). The programs forLO 5 through LO 8 concern matrices. The program for LO 9 is explained there.

Array objects

Concept An array is created in three steps: first a variable of an array type is declared; then the array is allocated; finally, theelements of the array are given values. The syntax for accessing an array a is a[i] , and the field a.length gives the length of the array, so that if we modify the program by changing the sizeof the array the rest of the program need not change.

Program: Array01A.java

// Learning Object Array01A //    array objectspublic class Array01A {     public static void main(/*String[] args*/) {         int[] fib;         fib = new int[7];         fib[0] = 0;         fib[1] = 1;         for (int i = 2; i < fib.length; i++)             fib[i] = fib[i-1] + fib[i-2];     }}

The program creates an array in the three steps described above.

  • Initially, the variable fib of type integer array (denoted int[] ) is allocated and contains the null value.
  • new fib[7] creates an array object with its seven fields having the default integer value zero; then the reference to the object is returned and stored in the variable fib .
  • The length field of the array is displayed above the cells for the elements.
  • A for loop is used to assign values to each element of the array.
  • The thin white lines show the constants and expressions that are used as indices into the array.
  • Automatic dereferencing : Although expressions like fib[i-2] seem to indicate that fib is being indexed, fib contains a reference to an array; an implicit operation of dereferencing is carried out to obtain thearray itself from the reference and the index [i-2] is then applied to that array.

Concept It is possible to combine the first two steps in creating an array: declaring the array field and allocating the array object.

Program: Array01B.java

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Source:  OpenStax, Learning objects for java (with jeliot). OpenStax CNX. Dec 28, 2009 Download for free at http://cnx.org/content/col10915/1.2
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