Sort the ArrayList of custom objects by property? Recommended | Java Debug Notes

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Sort the ArrayList of custom objects by property?

I’ve read about sorting arrayLists using comparators, but compareTo of all the examples that people use, according to some research, is a method of String.

I want to sort the ArrayList by one of the attributes of the custom object: the Date object (getStartDay()). In general, I compare them, item1.getStartDate().before(item2.getStartDate()) so I wonder if I can write something like the following:

public class CustomComparator { public boolean compare(Object object1, Object object2) { return object1.getStartDate().before(object2.getStartDate()); } } public class RandomName { ... Collections.sort(Database.arrayList, new CustomComparator); . }Copy the code

Answer:

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Comparable, since the Date implementation, has the same method String as the compareTo method.

Therefore, your custom setting Comparator might look like this:

public class CustomComparator implements Comparator<MyObject> { @Override public int compare(MyObject o1, MyObject o2) { return o1.getStartDate().compareTo(o2.getStartDate()); }}Copy the code

The compare() method must return an int, so Boolean can’t just return one that you planned anyway.

Your sorting code is almost as you wrote it:

Collections.sort(Database.arrayList, new CustomComparator());
Copy the code

If you don’t need to reuse the comparator, a shorter way to write all this is to write it as an inline anonymous class:

Collections.sort(Database.arrayList, new Comparator<MyObject>() { @Override public int compare(MyObject o1, MyObject o2) { return o1.getStartDate().compareTo(o2.getStartDate()); }});Copy the code

Since the Java – 8

Now you can write the last example Comparator in a shorter form by using a lambda expression:

Collections.sort(Database.arrayList, 
                        (o1, o2) -> o1.getStartDate().compareTo(o2.getStartDate()));
Copy the code

And List provides a sort(Comparator) method, so you can shorten this time even further:

Database.arrayList.sort((o1, o2) -> o1.getStartDate().compareTo(o2.getStartDate()));
Copy the code

This is a very common idiom, so there is a built-in method for the Comparator to generate one for classes with Comparable keys:

Database.arrayList.sort(Comparator.comparing(MyObject::getStartDate));
Copy the code

All of these are equivalent forms.

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The author suggests: many kinds of ways, good envy!! A simple extension of the Comparable and Comparator calls from Collections#sort() uses the strategy pattern, which essentially means that we implement the provided methods when we want to use them

We can rewrite the sorting algorithm by implements Comparable

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