The data type for the Iterator interface must match the data type of the set. Using Java 5, our example would look something like what you see in Listing 3. For example, iterator method on java. As Set extends Collection Interface and Collection extends Iterable Interface, we can use for-each loop to loop through the keySet. It is really an interface that is part of the Collections class. Today both Vector and Hashtable are generic classes, but back then generics were not part of the Java language. This method takes a single parameter that is a functional interface. Though foreach loop internally uses Iterator for traversing elements, It doesn't expose handle to that Iterator, which means you only have remove method of Map to remove entries. Note that for some collection, such as List, we still can use the index to traverse normally. The Iterator interface provides us with a method, hasNext , which returns false if there are no more members. I asked after doing my best to make sense of the documentation and wondering if I missed anything.
Java Sets A Set in Java is a collection of items, each of the same data type, and none of which are duplicates. This lesson will cover two major sets: one is ordered, and the other is not. Set is part of a larger Java class, called Collections. Set is an interface in the collections class, but in order to use any of the methods within it, we need to import the Java utility packages. This code must go before any class declaration in your code: import java. We will be creating a HashSet, which is a truly unordered set. Also, a TreeSet will maintain items in ascending order. In our example, we'll start with a HashSet and then loop through it. This code shows how to create a new Set using the Set interface. We can quickly walk through each item in the Set and display the output. When the program runs, we get a iterate through set java of our trees, as seen here: Notice how the order doesn't match the order we entered the trees. This highlights the unordered nature of the HashSet. In fact, until we compile and run the program, we really won't know what order the list will be in. This does not mean the order is random. The order is actually determined by Java. Once the set is built and compiled, the order or un-order, as it were is set. You don't have control over it, but you could run and re-run the code and get the same un order. There is another loop we can use, known as the 'forEach loop,' which is a variation of the 'for' loop. It is only in Java versions 8 and newer, so if you are on an older version, this will not work. The loop uses the forEach method and is accessed by first referencing the Set trees. You can then display the output, though the code is a little different than we've seen before. We will make use of the Java Iterator interface, which lets you cycle through lists and sets. In order to use it, you need to create a new instance of the Iterator. Next, we make use of a 'while' loop that runs while the set has a next item to look for. The Iterator interface provides us with a method, hasNextwhich returns false if there are no more members. Inside the 'while' loop, we display some info and the value from the set. To get the value, the method is next. The data type for the Iterator interface must match the data type of the set. Remember that our output for the HashSet was somewhat random. Again, we will create the same list, except the set is now a TreeSet, and then loop over the set using Iterator. Also, we moved a few trees around so they would not be in any order. If you use HashSet or TreeSet, the code is the same to loop through them, just be aware the TreeSet will be in ascending order. Lesson Summary To recap, a Set in Java is a collection of objects. It is really an interface that is part of the Collections iterate through set java. A HashSet is unordered, while a TreeSet is stored in ascending order. We can iterate over these sets using a 'for' loop, a 'forEach' loop, or by making use of the Iterator interface, which lets you cycle through lists and sets. Iterator provides methods for determining the next element and accessing elements in the set.
