Sorting an Array

Arrays

Creating an Array

Accessing an Array

Looping over an Array

Filtering an Array

Sorting an Array

Mapping an Array

Reducing an Array

You can sort an array in Java with java.util.Arrays.sort

Sort an array

The static method java.util.Arrays.sort can be used if the sort is simple:

Sort the list 2,7,5,4.

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class P{ public static void main(String [] argv){ int [] x = {2,7,5,4}; java.util.Arrays.sort(x); for (int i=0;i<x.length;i++) System.out.println(x[i]); } }

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Sort an array using a custom comparator

You can create your own comparator. This means that you decide how the sort is to be done. You must implement the compare method - this returns a negative, zero or a positive integer depending on how o1 compares to o2.

return -1 (or any negative number) if o1 comes before o2
return  0 if o1 and o2 are the same
return  1 (or any positive number) if o1 comes after o2

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class P{ public static void main(String [] argv){ String []a= {"one","two","three"}; java.util.Arrays.sort(a,new C()); for (int i=0;i<a.length;i++) System.out.println(a[i]); } } class C implements java.util.Comparator<String>{ public int compare(String o1, String o2){ return -1*o1.compareTo(o2); } }

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Sort an array using an anonymous custom comparator

This is essentially the same as the above - but you do not need to explicitly name the comparator.

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class P{ public static void main(String [] argv){ //Create the test array String []a= {"one","two","three","four","five"}; //Sort using an anonymous comparator java.util.Arrays.sort(a, new java.util.Comparator<String>(){ public int compare(String o1, String o2){ return -1*o1.compareTo(o2); } }); //Print the result to show it has been sorted for (int i=0;i<a.length;i++) System.out.println(a[i]); } }

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Sort an ArrayList

For an ArrayList (or any List) you use Collections.sort

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import java.util.ArrayList; import java.util.Collections; class P{ public static void main(String [] argv){ ArrayList<String> x = new ArrayList<String>(); x.add("one");x.add("two"); x.add("three");x.add("four"); Collections.sort(x); for (String s:x) System.out.println(s); } }

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Sort an ArrayList using a Custom Comparator

In this example you can sort an ArrayList of List. Notice that the output is ordered by region:

[[Angola, Africa, 1250000, 14500000, 14935000000],
 [Benin, Africa, 112622, 7100000, 3763000000],
 [Botswana, Africa, 581730, 1800000, 7812000000],
 ...

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import java.util.*; import java.io.*; public class P{ public static void main(String[]argv) throws Exception{ ArrayList<List<String>> world = new ArrayList<List<String>>(); String line; BufferedReader fh = new BufferedReader(new FileReader("bbc.txt")); while (null!=(line=fh.readLine())) world.add(Arrays.asList(line.split("\t"))); Comparator<List<String>> mc = new Comparator<List<String>>(){ public int compare(List<String> o1, List<String> o2){ return o1.get(1).compareTo(o2.get(1)); } }; Collections.sort(world,mc); System.out.println(world.toString().replace("],","],\n")); } }

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