Programming Homework Help

I’m working on a java question and need guidance to help me understand better.

Is about autocomplete, 3 small classes need to be done 

Class 1

public class Term implements Comparable<Term> { /**
    * Initialize a term with the given query and weight.
    * This method throws a NullPointerException if query is null,
    * and an IllegalArgumentException if weight is negative.
    */ public Term(String query, long weight) { } /**
    * Compares the two terms in descending order of weight.
    */ public static Comparator<Term> byDescendingWeightOrder() { } /**
    * Compares the two terms in ascending lexicographic order of query,
    * but using only the first length characters of query. This method
    * throws an IllegalArgumentException if length is less than or equal
    * to zero.
    */ public static Comparator<Term> byPrefixOrder(int length) { } /**
    * Compares this term with the other term in ascending lexicographic order
    * of query.
    */ @Override public int compareTo(Term other) { } /**
    * Returns a string representation of this term in the following format:
    * query followed by a tab followed by weight
    */ @Override public String toString(){ } } 

Class 2

public class BinarySearch { /**
    * Returns the index of the first key in a[] that equals the search key,
    * or -1 if no such key exists. This method throws a NullPointerException
    * if any parameter is null.
    */ public static <Key> int firstIndexOf(Key[] a, Key key, Comparator<Key> comparator) { } /**
    * Returns the index of the last key in a[] that equals the search key,
    * or -1 if no such key exists. This method throws a NullPointerException
    * if any parameter is null.
    */ public static <Key> int lastIndexOf(Key[] a, Key key, Comparator<Key> comparator) { } 

Class 3

public class Autocomplete { /**
* Initializes a data structure from the given array of terms.
* This method throws a NullPointerException if terms is null.
*/ public Autocomplete(Term[] terms) { } /**
* Returns all terms that start with the given prefix, in descending order of weight.
* This method throws a NullPointerException if prefix is null.
*/ public Term[] allMatches(String prefix) { } }