Cannot Collections.binarySearch() a collection that implements Comparable.

[O. Ziggy]

Im struggling to understand why these two statements are generating a compiler error

System.out.println("Search London in a Comparable object - " + Collections.binarySearch(cities, "London"));
System.out.println("Search London in a Comparable object with the comparator - " + Collections.binarySearch(cities2, "London",cc));

The code used is shown below

class Cities implements Comparable<Cities>{
	String title;
	
	Cities(String aTitle){
		this.title = aTitle;
	}
	
	public String getTitle(){
		return title;
	}

	public String toString(){
		return "Title : " + title;
	}

	public int compareTo(Cities c){
		return title.compareTo(c.getTitle());
	}
}

class CitiesNoComparable {
	String title;
	
	CitiesNoComparable(String aTitle){
		this.title = aTitle;
	}
	
	public String getTitle(){
		return title;
	}

	public String toString(){
		return "Title : " + title;
	}

}

class TitleSortComparator implements Comparator<CitiesNoComparable>{
	public int compare(CitiesNoComparable a, CitiesNoComparable b){
		return a.getTitle().compareTo(b.getTitle());
	}
}

And the instances are initialised as follows

	List<Cities> cities = new ArrayList<Cities>();
		cities.add(new Cities("London"));
		cities.add(new Cities("Manchester"));
		cities.add(new Cities("Luton"));
		cities.add(new Cities("Wimbledon"));
		cities.add(new Cities("Birmingham"));
		cities.add(new Cities("Isle of Wight"));		
		Collections.sort(cities);

		List<CitiesNoComparable> cities2 = new ArrayList<CitiesNoComparable>();
		
		cities2.add(new CitiesNoComparable("London"));
		cities2.add(new CitiesNoComparable("Manchester"));
		cities2.add(new CitiesNoComparable("Luton"));
		cities2.add(new CitiesNoComparable("Wimbledon"));
		cities2.add(new CitiesNoComparable("Birmingham"));
		cities2.add(new CitiesNoComparable("Isle of Wight"));

		TitleSortComparator cc	= new TitleSortComparator();
		Collections.sort(cities2, cc);

The error being generated is shown below

ScjpSamples.java:1695: cannot find symbol
symbol  : method binarySearch(java.util.List<Cities>,java.lang.String)
location: class java.util.Collections
                System.out.println("Search London in a Comparable object - " + Collections.binarySearch(cities, "London"));
                                                                                          ^
ScjpSamples.java:1696: cannot find symbol
symbol  : method binarySearch(java.util.List<CitiesNoComparable>,java.lang.String)
location: class java.util.Collections
                System.out.println("Search London in a Comparator object without the comparator - " + Collections.binarySearch(cities2, "London"));
                                                                                                                 ^
ScjpSamples.java:1697: cannot find symbol
symbol  : method binarySearch(java.util.List<CitiesNoComparable>,java.lang.String,TitleSortComparator)
location: class java.util.Collections
                System.out.println("Search London in a Comparable object with the comparator - " + Collections.binarySearch(cities2, "London",cc));
                                                                                                              ^
3 errors

- Error 1 is complaining about the cities object. The Cities class implements Comparable so why is it not allowing it
- The second error is complaining about cities2. This is based on the CitiesNoComparable class. This class does not implement comparable so this error is probably justified.
- The third error is based on the same object but i am passing a Comparator in the search. Shouldnt this have been accepted?

Thanks

[Henry Wong]

O. Ziggy wrote:
- Error 1 is complaining about the cities object. The Cities class implements Comparable so why is it not allowing it
- The second error is complaining about cities2. This is based on the CitiesNoComparable class. This class does not implement comparable so this error is probably justified.
- The third error is based on the same object but i am passing a Comparator in the search. Shouldnt this have been accepted?

It's not complaining about the Cities or CitiesNoComparable class. It's complaining about key parameter to the binarySearch method. This parameter is the same type as the generic (or super); and in both cases, you provided a string, ie. "London". It needs to be either a Cities or CitiesNoComparable instance.

Henry

[O. Ziggy]

oh yes ofcourse. I was supposed to search for an object with "London" as its city. Not the actual city itself as a string.

	System.out.println("Search London in a Comparable object - " + Collections.binarySearch(cities, new Cities("London")));
	System.out.println("Search London in a Comparable object with the comparator - " + Collections.binarySearch(cities2, new CitiesNoComparable("London"),cc));

Thanks