Category Archives: Java

1863. Sum of All Subset XOR Totals

key idea is how to generate all subset of array.

take 3 elements of arrays for example,

[a, _, _] -> [1, 0, 0]
[_, b, _] -> [0, 1, 0]
[a, b, _] -> [1, 1, 0]
[_, _, c] -> [0, 0, 1]
[a, _, c] -> [1, 0, 1]
[_, b, c] -> [0, 1, 1]
[a, b, c] -> [1, 1, 1]

integer i is loop through 1 to Math.pow(2, nums.length) – 1, for each integer, we check how many 1 bit

i & 1, if it is 1, we get nums[index] out as elements for subarray.

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class Solution {
    public int subsetXORSum(int[] nums) {
        int result = 0;
    	int x = (int)Math.pow(2, nums.length) - 1;
        for(int i=1;i<=x;i++) {
        	int p = i;
        	int r = 0;
        	for(int j=0;j<nums.length;j++) {
        		int t = p & 1;
        		p >>= 1;
 
        		if (t == 1) {
    				r ^= nums[j];
        		}
        	}
 
        	result += r;
        }
 
        return result;
    }
}

java – Spring Mergeable

There are 4 type of Mergeable Object in Spring

ManagedList
ManagedProperties
ManagedMap
ManagedSet

Take ManagedList as example

if mergeEnabled is set to be false, merge will stop
if parent is null, return itself.
and finally it will add parent firstly and then add children.

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	@Override
	@SuppressWarnings("unchecked")
	public List<E> merge(@Nullable Object parent)
	{
		if (!this.mergeEnabled) {
			throw new IllegalStateException("Not allowed to merge when the 'mergeEnabled' property is set to 'false'");
		}
		if (parent == null) {
			return this;
		}
		if (!(parent instanceof List)) {
			throw new IllegalArgumentException("Cannot merge with object of type [" + parent.getClass() + "]");
		}
		List<E> merged = new ManagedList<>();
		merged.addAll((List<E>) parent);
		merged.addAll(this);
		return merged;
	}

java – method bridge

java reflection could get all methods of a class.

isBridge() could check whether method is bridged or not.

when we override parent class general type method in child class, method will become bridged.

here is example:

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package org.spring.main;
 
import java.lang.reflect.Method;
 
import org.springframework.core.BridgeMethodResolver;
 
public class BridgeMethodResolverEntry {
 
	public class Node<T> {
 
	    public T data;
 
	    public Node(T data) { this.data = data; }
 
	    public void setData(T data) {
	        System.out.println("Node.setData");
	        this.data = data;
	    }
	}
 
	public class MyNode extends Node<Integer> {
 
	    public MyNode(Integer data) { super(data); }
 
	    @Override
	    public void setData(Integer data) {
	        System.out.println("MyNode.setData");
	        super.setData(data);
	    }
	}
 
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		BridgeMethodResolverEntry x = new BridgeMethodResolverEntry();
		MyNode resolver = x.new MyNode(5);
		Method[] declaredMethods = resolver.getClass().getDeclaredMethods();
 
		for (int i = 0; i < declaredMethods.length; i++) {
		            Method declaredMethod = declaredMethods[i];
		            String methodName = declaredMethod.getName();
		            Class<?> returnType = declaredMethod.getReturnType();
		            Class<?> declaringClass = declaredMethod.getDeclaringClass();
		            boolean bridge = declaredMethod.isBridge();
		            System.out.print((i+1) + " method name is" + methodName + ", return type is " + returnType + "  ");
		            System.out.print(bridge ? " is Bridge Method" : " is not Bridge Method");
		            System.out.println("  it is declared at "+declaringClass.getSimpleName()+"");            
		}                        
 
	}
 
}

Java – Static Initialization Block

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package com.dw.lib.test;
 
public class StaticInitializationBlock {
 
	static  
    {  
        System.out.println("STATIC BLOCK");  
    } 
 
	public StaticInitializationBlock() {
		System.out.println("constructor");
	}
 
	public static void main(String[] args) {
 
		StaticInitializationBlock block = new StaticInitializationBlock();
 
		System.out.println("MAIN METHOD");
 
	}
 
}

output:
STATIC BLOCK
constructor
MAIN METHOD

log4j src – parse log4j.xml building DocumentBuilder

log4j uses DocumentBuilder to parse log4j.xml

here is example code which is same as log4j original source code.

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package main;
 
import java.io.*;
import java.net.*;
 
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
 
import org.w3c.dom.Document;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import org.xml.sax.InputSource;
import org.xml.sax.SAXException;
 
public class DocumentBuilderEntry {
 
	public static void loop(Node node) {
	    // do something with the current node instead of System.out
	    System.out.println(node.getNodeName());
 
	    NodeList nodeList = node.getChildNodes();
	    for (int i = 0; i < nodeList.getLength(); i++) {
	        Node currentNode = nodeList.item(i);
	        if (currentNode.getNodeType() == Node.ELEMENT_NODE) {
	            //calls this method for all the children which is Element
	        	loop(currentNode);
	        }
	    }
	}
 
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		URL url = DocumentBuilderEntry.class.getResource("log4j.xml");
		DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance(); 
		try {
			DocumentBuilder parser = factory.newDocumentBuilder();
			URLConnection uConn = url.openConnection();
 
			uConn.setUseCaches(false);
			InputStream stream = uConn.getInputStream();
			try {
			  InputSource src = new InputSource(stream);
			  src.setSystemId(url.toString());
			  Document doc = parser.parse(src);
			  loop(doc.getDocumentElement());
			} catch (SAXException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} finally {
			  stream.close();
			}
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (ParserConfigurationException e1) {
			// TODO Auto-generated catch block
			e1.printStackTrace();
		}
 
 
 
	}
 
}

Spring – MultiValueMap and LinkedMultiValueMap

MultiValueMap is common object type in Spring Utils source code.

Here is an example of how to use it.

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MultiValueMap<String, String> multiValueMaps = new LinkedMultiValueMap<String, String>();
multiValueMaps.add("Tom", "Book");
multiValueMaps.add("Tom", "Pen");
 
multiValueMaps.add("ABC", "Company");
multiValueMaps.add("ABC", "WebSite");
 
for(String key: multiValueMaps.keySet()) {
	List<String> value = multiValueMaps.get(key);
	System.out.print(key + "\t");
	for(String s: value) {
		System.out.print(s + "\t");
	}
	System.out.println();
}

Java – Runtime.getRuntime().addShutdownHook()

a useful method when program done and run last step task.

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public class AddShutdownHookTest {
 
	static class Message extends Thread {
 
		public void run() {
			System.out.println("Bye.");
		}
	}
 
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		try {
 
			// register Message as shutdown hook
			Runtime.getRuntime().addShutdownHook(new Message());
 
			// print the state of the program
			System.out.println("Program is starting...");
 
			// cause thread to sleep for 3 seconds
			System.out.println("Waiting for 3 seconds...");
			Thread.sleep(3000);
 
			// print that the program is closing
			System.out.println("Program is closing...");
 
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

Tomcat Src – tomcat logging

This is starting of tomcat source code analysis.

Tomcat Log is simple.

the following is main example on how to invoke tomcat logging

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package main;
 
import org.apache.juli.logging.Log;
import org.apache.juli.logging.LogFactory;
 
public class tomcat {
	public static void main(String[] args) {
		Log log = LogFactory.getLog(tomcat.class);
		log.info("this is a info message from tomcat");
		log.debug("this is a debug message from tomcat");
	}
}

but when debugging tomcat src, I find that org.apache.juli.ClassLoaderLogManager is invoked.

Its entry is in catalina.sh as starting up script for tomcat.

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-Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager

Anypoint Studio Frozen Problem when starting on Mac Platform

Anypoint Studio Frozen Problem when starting on Mac Platform

solution: most important is to install jdk 1.8.0 151, this is max version of anypoint studio that support.
any version older that 151 would fail to work.

Edit target AnypointStudio.ini
AnypointStudio.app/Contents/Eclipse/AnypointStudio.ini

use
-vm
/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/bin
to point to JAVA_HOME of jdk 1.8 151

use
-clean to clean up workspace

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-clean
-startup
../Eclipse/plugins/org.eclipse.equinox.launcher_1.3.100.v20150511-1540.jar
-vm
/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/bin
--launcher.library
../Eclipse/plugins/org.eclipse.equinox.launcher.cocoa.macosx.x86_64_1.1.300.v20150602-1417
-vmargs
-Xms512m
-Xmx1536m
-XX:MaxPermSize=512m
-Dosgi.instance.area.default=@user.home/AnypointStudio/workspace
-Dhttps.protocols=TLSv1.1,TLSv1.2
-Djava.awt.headless=true
-XstartOnFirstThread
-Dorg.eclipse.swt.internal.carbon.smallFonts

Spring Tomcat – integration error

Spring with Tomcat

add spring security, then restart tomcat get the following error.

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Caused by: java.util.zip.ZipException: invalid LOC header (bad signature)
	at java.util.zip.ZipFile.read(Native Method)
	at java.util.zip.ZipFile.access$1400(ZipFile.java:60)
	at java.util.zip.ZipFile$ZipFileInputStream.read(ZipFile.java:717)
	at java.util.zip.ZipFile$ZipFileInflaterInputStream.fill(ZipFile.java:419)
	at java.util.zip.InflaterInputStream.read(InflaterInputStream.java:158)
	at sun.misc.IOUtils.readFully(IOUtils.java:65)
	at java.util.jar.JarFile.getBytes(JarFile.java:425)
	at java.util.jar.JarFile.getManifestFromReference(JarFile.java:193)
	at java.util.jar.JarFile.getManifest(JarFile.java:180)
	at org.apache.catalina.webresources.AbstractSingleArchiveResourceSet.initInternal(AbstractSingleArchiveResourceSet.java:140)
	... 13 more

try to build on console.

$ mvn package.

if mvn says some jar/war is interuptted,
then delete these files and let mvn to download them again

Spring 5 has to default a passwordEncoder

However, if passwordEncoder is set to be NoOpPasswordEncoder to ‘ignore’ passwordEncorder

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<bean id="passwordEncoder"
class="org.springframework.security.crypto.password.NoOpPasswordEncoder"
factory-method="getInstance">