Arrays class

Some methods below: asList(...), binarySearch(...), equals(...), fill(...), sort(...)

■ The Arrays class lives in java.util. It provides utilities for manipulating arrays, just as the Collections class provides utilities for manipulating collections.

■ Another handy and important array manipulation method happens to live elsewhere, in the System class. It is arrayCopy(...) .

List asList( Obj[ ] ) method

■ Creating Lists from arrays. This method converts an Array of objects to a List. (Note: not an array of primitives.)

■ The opposite of .asList(...) is the .toArray(...) method found in every Collection class. (see toArray(...) )

■ It returns a fixed-size List eligible for the use of List methods, backed by data in the array.

■ It doesn’t actually copy the array's data into the List. It changes to the List “write through” mode to the array, changing the List when the array is changed. i.e.

import java.lang.reflect.Array;

import java.util.*;

public class TestArray {

public static void main(String args[ ]) {

String[ ] array = {"You", "Me", "Her", "Him"};

List ourList;

ourList = Arrays.asList(array) ;

System.out.println("List length: " + ourList.size());

System.out.println("List contains: " + ourList);

System.out.println("Array length: " + array.length);

for (int i =0; i<Array.getLength(array); i++) System.out.println("Array is: " + Array.get(array, i));

System.out.println(" ");

ourList.set( 1, "XXXXX"); // change AN OBJECT in THE LIST

//ourList.add("YYYYY"); // ADD AN OBJECT TO THE LIST

System.out.println("List length now: " + ourList.size());

System.out.println("List now contains: " + ourList);

System.out.println("Array length now: " + array.length);

for (int i =0; i<Array.getLength(array); i++) System.out.println("Array is now: " + Array.get(array, i));

}

}

int binarySearch( type[ ], key ) method

■ The target array must already be sorted or you get undefined results. No error is thrown however.

■ If multiple results are possible, you may get any or none of them. No error however.

■ It returns the int index of the found item, if one is found.

■ It always returns a negative int if something is not found. It returns the negative of the desired key's correct prospective insertion point, minus 1..

boolean equals( array1[ ], array2[ ] ) method

■ It returns a boolean. Works for all types of arrays, both of primitives and objects.

■ It overrides equals(...) for arrays.

■ Comparing two null array references are always considered equal and will return true.

■ This method produces the same results as the following code:

if( java.util.Arrays.equals( array1, array2 ) );

void fill( array[ ], value ) method

■ fill(...) has versions for all the primitive and boolean array types, plus objects.

■ It puts the specified value into each cell of the specified array.

void sort( array[ ], from, to ) method

■ sort(...) provides versions for all the primitive and boolean array types, plus objects.

■ The from index starts at zero and the to index goes up to position to minus one.

i.e. This code prints the ascending sorted sequence 1 2 3 4

import java.util.*;

List aList;

String[ ] array = {"4", "3", "2", "1"};

Arrays.sort (array, 0, 4);

List aList = Arrays.asList(array) ;

System.out.println(aList);

■ There is no sorting if from = to. No error is returned.

■ Any equal elements which are found are not reordered.

■ Both the int from and int to are optional.

i.e. Arrays.sort (array); works the same as the Arrays.sort (array, 0, 4); example shown above.

■ sort(Obj[ ], Comparator) and sort(Obj[ ], from, to, Comparator) both let you name a comparator.

■ To sort and reverse the order of an array, use this: Arrays.sort(yourarray, Collections.reverseOrder( ));