You may know how to use LISTAGG() on a single table, but don’t know how to use LISTAGG on multiple joined tables. This example demonstrates how to use aggregate function on joined multiple tables in Oracle 12g. Assuming we have the following two tables. “User” Table ID Name 111 aaa 222 bbb 333 ccc “Record” … 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
The following are different ways of reading a txt file line by line in linux shell. To fun the following scripts, you should create a “test.txt” file under the same directory with your script. Method 1 Use file names as parameter to the shell script. readFile.sh #!/bin/bash while read line do name=$line echo “Text read … Read more
When using ASTParser, we often need to find type of variable in the code. This information is contains in the binding. The following code shows how to write a stand alone ASTParser to parse Java file and get the type information of variables in the code. The target code to parse: package test; import … Read more
The flatMap() method can be used for combining a stream of streams to one stream of objects. In this post I will show how to use flatMap() by using several use cases. Use Case 1 Suppose you have a list of orders and each order contains a list of items. class Order{ String orderId; ArrayList<Item> … 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
The filter() method is an intermediate operation. It returns a stream consisting of the elements of this stream that match the given condition. import java.util.ArrayList; import java.util.Arrays; import java.util.List; import java.util.stream.Stream; public class Java8Filter<T> { public static void main(String[] args) { List<String> list = new ArrayList<String>(); list.add("java"); list.add("php"); list.add("python"); list.add("lisp"); list.add("c++"); … Read more
The map() method is an intermediate operation. It returns a stream consisting of the results of applying the given function to each element in the stream. The following code returns a stream of Integers, which are results of applying String.length() method. import java.util.ArrayList; import java.util.List; import java.util.stream.Stream; public class Java8Map { public … Read more
1. From Arrays String[] arr = { "program", "creek", "program", "creek", "java", "web", "program" }; stream = Stream.of(arr);String[] arr = { "program", "creek", "program", "creek", "java", "web", "program" }; stream = Stream.of(arr); stream = Stream.of("program", "creek", "program", "creek", "java", "web", "program");stream = Stream.of("program", "creek", "program", "creek", "java", "web", "program"); String[] stringArr = {"a", "b", "c", … Read more