JAVA for Beginners
/* Methods to access alpha. Members of a class can access a
private member of the same class.
*/
void setAlpha(int a) {
alpha = a;
int getAlpha() {
return alpha;
Main Program:
class AccessDemo {
public static void main(String args[]) {
MyClass ob = new MyClass();
ob.setAlpha(-99);
System.out.println("ob.alpha is " + ob.getAlpha());
// You cannot access alpha like this:
// ob.alpha = 10; // Wrong! alpha is private!
// These are OK because beta and gamma are public.
ob.beta = 88;
ob.gamma = 99;
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Another example using arrays:
class FailSoftArray {
private int a[]; // reference to array
private int errval; // value to return if get() fails
public int length; // length is public
/* Construct array given its size and the value to
return if get() fails. */
public FailSoftArray(int size, int errv) {
a = new int[size];
errval = errv;
length = size;
// Return value at given index.
public int get(int index) {
if(ok(index)) return a[index];
return errval;
// Put a value at an index. Return false on failure.
public boolean put(int index, int val) {
if(ok(index)) {
a[index] = val;
return true;
return false;
// Return true if index is within bounds.
private boolean ok(int index) {
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if(index >= 0 & index < length) return true;
return false;
Main Program:
class FSDemo {
public static void main(String args[]) {
FailSoftArray fs = new FailSoftArray(5, -1);
int x;
// show quiet failures
System.out.println("Fail quietly.");
for(int i=0; i < (fs.length * 2); i++)
fs.put(i, i*10);
for(int i=0; i < (fs.length * 2); i++) {
x = fs.get(i);
if(x != -1) System.out.print(x + " ");
System.out.println("");
// now, handle failures
System.out.println("\nFail with error reports.");
for(int i=0; i < (fs.length * 2); i++)
if(!fs.put(i, i*10))
System.out.println("Index " + i + " out-of-bounds");
for(int i=0; i < (fs.length * 2); i++) {
x = fs.get(i);
if(x != -1) System.out.print(x + " ");
else
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System.out.println("Index " + i + " out-of-
bounds");
Predicted Output:
Fail with error reports.
Index 5 out-of-bounds
Index 6 out-of-bounds
Index 7 out-of-bounds
Index 8 out-of-bounds
Index 9 out-of-bounds
0 10 20 30 40 Index 5 out-of-bounds
Index 6 out-of-bounds
Index 7 out-of-bounds
Index 8 out-of-bounds
Index 9 out-of-bounds
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JAVA for Beginners
Arrays and Strings
Arrays
An array can be defined as a collection of variables of the same type defined by a common name,
e.g. an array called Names which stores the names of your class mates:
Names *name , name , name , … nameX+
Arrays in Java are different from arrays in other programming languages because they are
implemented as objects.
One-dimensional Arrays
Declaration: type array-name[ ] = new type[size];
e.g. int sample[] = new int[10];
The following code creates an array of ten integers, fills it up with numbers using a loop and then
prints the content of each location (index) of the array:
class ArrayDemo {
public static void main(String args[]) {
int sample[] = new int[10];
int i;
for(i = 0; i < 10; i = i+1)
sample[i] = i;
for(i = 0; i < 10; i = i+1)
System.out.println("This is sample[" + i + "]: " +
sample[i]);
Predicted Output:
This is sample[0]: 0
This is sample[1]: 1
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This is sample[2]: 2
This is sample[3]: 3
This is sample[4]: 4
This is sample[5]: 5
This is sample[6]: 6
This is sample[7]: 7
This is sample[8]: 8
This is sample[9]: 9
The following program contains two loops used to identify the smallest and the largest value stored
in the array:
class MinMax {
public static void main(String args[]) {
int nums[] = new int[10];
int min, max;
nums[0] = 99;
nums[1] = -10;
nums[2] = 100123;
nums[3] = 18;
nums[4] = -978;
nums[5] = 5623;
nums[6] = 463;
nums[7] = -9;
nums[8] = 287;
nums[9] = 49;
min = max = nums[0];
for(int i=1; i < 10; i++) {
if(nums[i] < min) min = nums[i];
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if(nums[i] > max) max = nums[i];
System.out.println("min and max: " + min + " " +
max);
Predicted Output:
min and max: -978 100123
Sorting an Array The Bubble Sort
The Bubble sort is one type, the simplest, of sorting algorithms. It can be used to sort data stored in
arrays but it is not ideal when the size of the array to sort is large.
class Bubble {
public static void main(String args[]) {
int nums[] = { 99, -10, 100123, 18, -978,
5623, 463, -9, 287, 49 };
int a, b, t;
int size;
size = 10; // number of elements to sort
// display original array
System.out.print("Original array is:");
for(int i=0; i < size; i++)
System.out.print(" " + nums[i]);
System.out.println();
// This is the Bubble sort.
for(a=1; a < size; a++)
for(b=size-1; b >= a; b -) {
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if(nums[b-1] > nums[b]) { // if out of order
// exchange elements
t = nums[b-1];
nums[b-1] = nums[b];
nums[b] = t;
// display sorted array
System.out.print("Sorted array is:");
for(int i=0; i < size; i++)
System.out.print(" " + nums[i]);
System.out.println();
Predicted Output:
Original array is: 99 -10 100123 18 -978 5623 463 -9 287 49
Sorted array is: -978 -10 -9 18 49 99 287 463 5623 100123
Two-Dimensional Arrays:
A two dimensional array is like a list of one-dimensional arrays. Declaration is as follows:
int table[][] = new int[10][20];
This would create a table made up of 10 rows (index: 0 to 9) and 20 columns (index: 0 to 19). The
following code creates a table, 3 rows by 4 columns, and fills it up woth numbers from 1 to 12. Note
that at index [0][0] = 1, [0][1] = 2, [0][2] = 3, [0][3] = 4, [1][0] = 5, etc.
class TwoD {
public static void main(String args[]) {
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int t, i;
int table[][] = new int[3][4];
for(t=0; t < 3; ++t) {
for(i=0; i < 4; ++i) {
table[t][i] = (t*4)+i+1;
System.out.print(table[t][i] + " ");
System.out.println();
Different syntax used to declare arrays:
Consider the following: type[ ] var-name;
The square brackets follow the type specifier, not the name of the array variable. For example, the
following two declarations are equivalent:
int counter[] = new int[3];
int[] counter = new int[3];
The following declarations are also equivalent:
char table[][] = new char[3][4];
char[][] table = new char[3][4];
This alternative declaration form offers convenience when declaring several arrays at the same time.
int[] nums, nums2, nums3; // create three arrays
This creates three array variables of type int. It is the same as writing:
int nums[], nums2[], nums3[]; // also, create three arrays
The alternative declaration form is also useful when specifying an array as a return type for a
method:
int[] someMeth( ) { ...
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This declares that someMeth( ) returns an array of type int.
Array References:
Consider a particular array, nums1, and another array, nums2 and at one point in the code we assign
one array reference to the other, i.e. nums2 = nums1. Then every action on nums2 will be as if it
were on nums1 (nums2 reference is lost).
class AssignARef {
public static void main(String args[]) {
int i;
int nums1[] = new int[10];
int nums2[] = new int[10];
for(i=0; i < 10; i++)
nums1[i] = i;
for(i=0; i < 10; i++)
nums2[i] = -i;
System.out.print("Here is nums1: ");
for(i=0; i < 10; i++)
System.out.print(nums1[i] + " ");
System.out.println();
System.out.print("Here is nums2: ");
for(i=0; i < 10; i++)
System.out.print(nums2[i] + " ");
System.out.println();
nums2 = nums1; // nums2 is now nums1
System.out.print("Here is nums2 after assignment:
");
for(i=0; i < 10; i++)
System.out.print(nums2[i] + " ");
System.out.println();
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// now operate on nums1 array through nums2
nums2[3] = 99;
System.out.print("Here is nums1 after change through
nums2: ");
for(i=0; i < 10; i++)
System.out.print(nums1[i] + " ");
System.out.println();
Predicted Output:
Here is nums1: 0 1 2 3 4 5 6 7 8 9
Here is nums2: 0 -1 -2 -3 -4 -5 -6 -7 -8 -9
Here is nums2 after assignment: 0 1 2 3 4 5 6 7 8 9
Here is nums1 after change through nums2: 0 1 2 99 4 5 6 7 8 9
The Length Variable:
This variable automatically keeps track of the number of items an array can hold and NOT the actual
items. When an array is declared to have ten items, even if one does not put actual data, the length
of the array is 10. Something to keep in mind is also when dealing with two dimensional arrays. As
already explained in the theory lessons, two dimensional arrays are considered to be an array of
arrays. Hence calling up the length of such array would return the number of sub-arrays. To access
the length of the particular sub-array, one has to write the ‘row’ index, e.g. arrayname [0].length
// length of array demo
class LengthDemo {
public static void main(String args[]) {
int list[] = new int[10];
int nums[] = { 1, 2, 3 };
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int table[][] = { // a variable-length table
0 1 2 3
{1, 2, 3},
0 1 2 3
{4, 5}, 1 4 5
2 6 7 8 9
{6, 7, 8, 9}
};
System.out.println("length of list is " +
list.length);
System.out.println("length of nums is " +
nums.length);
System.out.println("length of table is " +
table.length); // returns number of rows
System.out.println("length of table[0] is " +
table[0].length);
System.out.println("length of table[1] is " +
table[1].length);
System.out.println("length of table[2] is " +
table[2].length);
System.out.println();
// using length to initialize list
for(int i=0; i < list.length; i++)
list[i] = i * i;
System.out.print("Here is list: ");
// using length to display list
for(int i=0; i < list.length; i++)
System.out.print(list[i] + " ");
System.out.println();
Predicted Output:
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