ComparatorGenerator

An easy-to-use helper class for generating a comparator for a specified class. Useful when sorting the instances of the specified class.

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##Features

A comparator is needed when sorting a list of elements of a specified class. But it is difficult to write a comparator, especially in multiple-criteria sorting.

This helper class makes it easy for you to obtain a comparator. What you need to do is just specifying some criteria and then the helper class will give you a comparator.

##Gradle

dependencies {
    compile 'xiaofei.library:comparator-generator:1.1'
}

##Maven

<dependency>
  <groupId>xiaofei.library</groupId>
  <artifactId>comparator-generator</artifactId>
  <version>1.1</version>
  <type>pom</type>
</dependency>

##Usage

Often, elements are sorted by the values of some specified fields or the return values of some specified methods. A field or a method has a corresponding criterion. A criterion has a priority. The field or the method with a higher priority will be considered first when sorting. A criterion has also a sorting order, which, by default, is ascending.

There are two ways to specify criteria: Using annotations and using APIs.

Using Annotations

Take the following class as an example:

class Item1 {
    @Criterion(priority = 3, order = Order.ASCENDING)
    private int price;
    @Criterion(priority = 2, order = Order.DESCENDING)
    private int distance;
    @Criterion(priority = 1) // The default sorting order is Order.ASCENDING.
    private String itemName;
}

Suppose there is an array named items1. Then we can sort the array as the following shows:

Comparator<Item1> comparator = new ComparatorGenerator<Item1>(Item1.class).generate();
Arrays.sort(items1, comparator);

Here is another example:

class Item3 {
	
    private int price;
    private int number;
    private String itemName;
    
    @Criterion(priority = 2, order = Order.ASCENDING)
    int getTotalPrice() {
        return price * number;
    }
    
    @Criterion(priority = 1) // The default sorting order is Order.ASCENDING.
    String getItemName() {
        return itemName;
    }
}

Suppose there is an array named items3. Then we can sort the array as the following shows:

Comparator<Item3> comparator = new ComparatorGenerator<Item3>(Item3.class).generate();
Arrays.sort(items3, comparator);

Using APIs

Take the following class as an example:

class Item2 {
	
    private int price;
	
    private int distance;
	
    private String itemName;
}

Suppose there is an array named items2. Then we can sort the array as the following shows:

Comparator<Item2> comparator2 =
    new ComparatorGenerator<Item2>(Item2.class)
        .addCriterion(3, "price", Order.ASCENDING)
        .addCriterion(2, "distance", Order.DESCENDING)
        .addCriterion(1, "itemName") // The default sorting order is Order.ASCENDING.
        .generate();
Arrays.sort(items2, comparator2);

Here is another example:

class Item4 {
	
    private int price;
    private int number;
    private int distance;
    
    int getTotalPrice() {
        return price * number;
    }
}

Suppose there is an array named items4. Then we can sort the array as the following shows:

Comparator<Item4> comparator4 =
    new ComparatorGenerator<Item4>(Item4.class)
        .addCriterion(3, "getTotalPrice", Order.ASCENDING)
        .addCriterion(2, "distance", Order.DESCENDING)
        .generate();
Arrays.sort(items4, comparator4);