Observability of the Java Virtual Machine

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The JVM is one of the most observable runtimes. It provides us lots of tools for troubleshooting a JVM application in production. 1. Thread observability Threads are how the JVM actually does work. When something is wrong in production, the symptom is almost always a thread: stopped, blocked, leaking etc. Thread dumps work on any JVM with no  instrumentation, no agents, no restarts. <Example project link with /threaddump endpoint>         // (1) Deadlock — two threads grab the same pair of locks in opposite order.         new Thread(() -> grab(LOCK_A, LOCK_B), "deadlock-A-then-B").start();         new Thread(() -> grab(LOCK_B, LOCK_A), "deadlock-B-then-A").start(); http://localhost:8080/actuator/threaddump To list the JVMS, we can use the command below. PS C:\observe-jvm> jps -lv 25296 jdk.jcmd/sun.tools.jps.Jps -Dapplication.home=C:\Program Files\Microsoft\jdk-21.0.3.9-hotspot -Xms8m -Djdk.module.main=...
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Using XOR operator in bit manipulation problems

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Problem description from LeetCode:

Given an array of numbers nums, in which exactly two elements appear only once and all the other elements appear exactly twice. Find the two elements that appear only once.

 Input: [1,2,1,3,2,5] 
Output: [3,5]

Solution:

If we apply XOR to all the elements, the result is the XOR of the lonely integers. The next step is finding the two integers that produce the XOR result. 
To do that, we can use this trick:
The set bits of A XOR B  gives us which bits are different in A and B
After finding the last different set bit, we can dissect the numbers from the input. 

Source Code:

class Solution {
    
   public static int[] singleNumber(int[] nums) {
        int xor = 0;
        for (int num : nums) {
            xor ^= num;
        }
        int lastSetBit = lastSetBit(xor);

        int set1 = 0; int set2 = 0;
        for (int i = 0; i < nums.length; ++i) {

            if ((nums[i] & lastSetBit) == 0) {
                set1 ^= nums[i];
            } else {
                set2 ^= nums[i];
            }
        }

        return new int[]{set1, set2};
    }

    private static int lastSetBit(int xor) {

        int i = 0;
        for (; i < 32; ++i) {
            if ((xor & (1 << i)) != 0) {
                break;
            }
        }

        return (1 << i);
    }
    
}


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