The following example shows how to compile and run Java program in command line mode with external jars. It is developed under Linux. 1. Compile & Run Java Program Without External Jar Let’s create a simple hello world program “helloworld.java”. public class helloworld{ public static void main(String[] args){ System.out.println("Hello!"); } }public class helloworld{ public static … Read more
Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses. For example, given n = 3, a solution set is: “((()))”, “(()())”, “(())()”, “()(())”, “()()()” Java Solution 1 – DFS This solution is simple and clear. In the dfs() method, left stands for the remaining number of (, right stands … Read more
To convert a Stream to an array, there is an overloaded version of toArray() method for Stream objects. The toArray(IntFunction<A[]> generator) method returns an array containing the elements of this stream, using the provided generator function to allocate the returned array. String[] stringArr = { "a", "b", "c", "d" }; Stream<String> stream = Stream.of(stringArr); String[] … Read more
What is Serialization? In Java, object serialization means representing an object as a sequence of bytes. The bytes includes the object’s data and information. A serialized object can be written into a file/database, and read from the file/database and deserialized. The bytes that represent the object and its data can be used to recreate the … Read more
Assuming you have the following list: List<String> list = Arrays.asList("a", "b", "c", "d", "e", "f");List<String> list = Arrays.asList("a", "b", "c", "d", "e", "f"); if you want to do something by using each element in a collection, there are two ways: 1) list.forEach(s -> System.out.println(s));list.forEach(s -> System.out.println(s)); 2) for(String s: list){ System.out.println(s); }for(String s: list){ System.out.println(s); … Read more
iterator often cause problems, because developers often do not know how it works. The following code is from source code of ArrayList. One common problem is throwing java.util.ConcurrentModificationException. This exception is actually throw in the remove method. remove() has to be called after next(). if remove() is called before next(), the size of arraylist changes, … Read more
1. What is Varargs in Java? Varargs (variable arguments) is a feature introduced in Java 1.5. It allows a method take an arbitrary number of values as arguments. public static void main(String[] args) { print("a"); print("a", "b"); print("a", "b", "c"); } public static void print(String … s){ for(String a: s) System.out.println(a); }public static void … Read more
Given the root of a tree, you are asked to find the most frequent subtree sum. The subtree sum of a node is defined as the sum of all the node values formed by the subtree rooted at that node (including the node itself). So what is the most frequent subtree sum value? If there … Read more
If you want to utilize Linux pipe advantages, you can do it in Java. You may want to do the following, which is wrong. String cmd = "ls" Process p = Runtime.getRuntime().exec(cmd);String cmd = "ls" Process p = Runtime.getRuntime().exec(cmd); The correct way of executing shell command is the following: String[] cmd = { "/bin/sh", "-c", … Read more
Given two strings of time, you may want to calculate the difference between them. The following provide a simple solution. import java.text.SimpleDateFormat; import java.util.Date; public class Main { public static void main(String[] args) throws Exception{ String time1 = "12:00:00"; String time2 = "12:01:00"; SimpleDateFormat format = new SimpleDateFormat("HH:mm:ss"); Date date1 = format.parse(time1); Date … Read more
Writing a counter in Java can be as simple as 2 lines. In addition to its simplicity, we can also utilize the parallel computation to increase the counter’s performance. import java.util.stream.Collectors; import java.util.stream.Stream; import java.util.Map; public class Java8Counter { public static void main(String[] args) { String[] arr = { "program", "creek", "program", "creek", "java", … Read more
In addition to Stream, java.util.stream package also provide a set of primitive type Streams such as DoubleStream, IntStream and LongStream. The primitive type streams are pretty similar with Stream. In this post I will use the IntStream to illustrate how to use primitive type streams. Create an IntStream From integer array: IntStream stream = IntStream.of(2, … Read more
1. Correct Way to Use Optional Consider the following example: List<String> list = new ArrayList<String>(); list.add("java"); list.add("php"); list.add("python"); list.add("perl"); list.add("c"); list.add("lisp"); list.add("c#"); Stream<String> wordStream = list.stream(); Stream<Integer> lengthStream = wordStream.map(s -> s.length()); Optional<Integer> sum = lengthStream.reduce((x, y) -> x + y); System.out.println(sum.get());List<String> list = new ArrayList<String>(); list.add("java"); list.add("php"); list.add("python"); list.add("perl"); list.add("c"); list.add("lisp"); list.add("c#"); Stream<String> … Read more
Instead of reducing a stream to a value, we can also collect the results. We can collect results to an array, a set/list, or a map by using the Stream.collect() method. 1. Collect Stream Results to Array List<String> list = new ArrayList<String>(); list.add("java"); list.add("php"); list.add("python"); Stream<String> wordStream = list.stream(); Stream<Integer> lengthStream = wordStream.map(s -> … Read more
The Java 8 Stream API contains a set of terminal operations (such as average, sum, min, max, and count) which return one value by combining the elements of a stream. Such terminal operations are called reduction operations. In addition to those terminal operations, the JDK also provides the general-purpose reduction method – reduce(), which this … Read more