Why Is HashCode Needed For Checking Duplicates In HashMap And Set
Jul 11, 2014
The keys in a HashMap and the values in a Set must all be unique, but this can be circumvented when using custom objects in a HashMap and Set, because the compiler has no way to determine if the objects are equal or not, as shown in the example below:
Java Code:
import java.util.LinkedHashMap;
import java.util.Map;
public class HashCodeEquals {
public void run(){
Person p1 = new Person(1, "John");
Person p2 = new Person(2, "Matt");
Person p3 = new Person(1, "John");
[code]....
Obviously the equals method is needed because that compares the two objects. But why is the hashCode method needed?
why interfaces are needed in Java,Now you saw what a class must do to avail itself of the s... - justpaste.it (if I paste the quote here, I get the "Page not found" error after posting -.^)
the first fragment reads that the compiler must be sure that a method exits at a compile time, whereas the second fragment denies it - if a[i] doesn't have the specified compareTo method, a JVM simply throws an exception.
method called []getLetterGrades but the only hint My professor told me was that I needed to declare another array list for this method and he wouldnt tell me anything else so bummer. But I don't understand why if what we are returning is a char. It would make sense to return an array list of char to get letter grade. Which is what i did but since the function is a char, the array list character wont work as a return.Primarily i would like to know the type that is needed. I just want an explanation for an array list in this method and how it would serve in this method.
Should we always use getters/setters, e.g. even in something like
Java Code:
class Foo { private static int foo; public int getFoo() { return foo; } public void setFoo(int fig) { foo = fig; } } mh_sh_highlight_all('java');
Or should we use the only when there is a good reason to use them, e.g. validation for a setter or may be computing a value to return from some variables etc. ?
I have a HTML page with an applet inside it.I have the applet class and all other needed classes in a jar file in jre/lib/ext folder (I'm on windows). But when I open the HTML file in a browser it gives me ClassNotFound exception..The HTML file is this:
I have this HostOnDemand class in its package in a jar file in jre/lib/ext but it can't be found.Since the applet is way too old, I'm using jre1.4.2 and netscape.
In the above program even if i comment out the Hashcode method , i believe it is still taking the memory address values from the native hashcode method of Object class. but the equals override implentation says that i have two insertions which are same . So as per my logic it should not allow the duplicate element to enter.but its not so ...the duplicate element is well inserted without hashcode .
hours - An array of seven integers to hold the number of hours worked by each employee.payRate - An array of seven doubles to hold each employee's hourly pay rate.wages - An array of seven doubles to hold each employee's gross wages.The class should relate the data in each array through the subscripts.
For example, the number in element 0 of the hours array should be the number of hours worked by the employee whose identification number is stored in element 0 of the employeeID array. That same employee's pay rate should be stored in element 0 of the payRate array. In addition to the appropriate accessor and mutator methods, the class should have a method that accepts an employee's identification number as an argument and returns the gross pay for that employee.Demonstrate the class in a complete program that displays each employee number and asks the user to enter that employee's hours and pay rate. It should then display each employee's identification number and gross wages.Input Validation: Do not accept negative values for hours or numbers less than 6.0 for a pay rate.
My problem with this program is that everytime I try to print the employee ID's or the wages, I get hashcode or something like it (#[I1a77cdc or something like that). I tried using the toString method, but it lists all of the values, when I'm trying to display one at a time. Here is the code for the class:
// moduleArray class public class moduleArray { final int NUM_EMPLOYEES = 7; int[] employeeID = {5658845, 4520125, 7895122, 8777541, 8451277, 1302850, 7580489}; int[] hours = new int[NUM_EMPLOYEES];
[code]...
This is the demo program to list the ID's. I've been messing with it for some time, and right now I just want it to display values.
import java.util.Scanner; public class moduleArrayDemo { public static void main(String[] args) { final int NUM_EMPLOYEES = 7; int[] ID = new int[NUM_EMPLOYEES];
I am studying Serialization from the SCJP 6 Kathy Sierra book. I came across this code snippet.
public class Cat implements Serializable { public static void main(String[] args) { Cat c = new Cat(); try { FileOutputStream fs = new FileOutputStream("testSer.ser"); ObjectOutputStream os = new ObjectOutputStream(fs); os.writeObject(c);
[code]....
The output is as follows.
files.Cat@4d43691d files.Cat@7f39ebdb
1) Why are the two hashcodes different?
2) Serialization is supposed to make and identical copy of any object and all its instance variables. So, if the hashcodes are different, are these objects located in different locations in heap?
Winston Gutkowski in one of the threads that one of the advatanges of String being immutable is that its hashcode could be cached. How does this work? I read somewhere that each String's hashcode is stored in a private int variable, but I don't understand how it is reused.
I'm trying to make a method that creates objects of a parameterized type randomly, but also to store the hashCode of different objects created and if at any time the percentage of different objects is less than 50% throw an exception.
This last part is where I've gotten stuck. I have created a population property where I store the different hashCodes and update it in the method adding the new hashCode from the new object. But I don't know how to do for to know if the percentage of different objects is less than 50%.
package fp.tipos.apps; import java.util.ArrayList; import java.util.Calendar; import java.util.GregorianCalendar; import java.util.List; import java.util.Random; public class FactoriaApps {
Let's say I'd like to print out something as a prompt without end with a new line char, so the standard System.out.println() is not an option and I'd like to find something else inplace of println().
Where to find that? I googled about that and learned printf() is the one I want, but how to get it from java api documentation.
I have a .jar that is installed in other computer(outside netbeans). Everything works fine except with report. I am sure the libraries needed for iReport is added in my lib folder but I receive this error message when I to load the report using the program.I use command prompt to see the error:
Java Code:
java -jar projCosting.jar mh_sh_highlight_all('java'); And here is the error message: Java Code: net.sf.jasperreports.engine.JRException: Error compiling report java source files : [b]D:CostingRptCosting_1288072391310_955208.java[/b] at net.sf.jasperreports.engine.design.JRJavacCompiler.compileClasses
[code]....
I have included the library in lib folder but do you think I have to install the iReport application?
I read this tutorial about overriding equal and hashcode method. [URL] ....
I understand how to override equal method, by overriding it, We can custom our compare. I also understand How to override hashcode, To make custom hash.
But still I can not understand why we do it? why if equal method override, we must override hashcode method too?If we don't what is the problem?
To honor the above contract we should always override hashCode() method whenever we override equals() method. If not, what will happen? If we use hashtables in our application, it will not behave as expected. As the hashCode is used in determining the equality of values stored, it will not return the right corresponding value for a key.
Is it the right reason in order to override:
Because when we customize equal method so it focus on special variables,We must change the hash code too in order to match with it, so hashcode also focus on those special variable.
I want to extend hashCode method in my class. As we know that hashCode is generating with 32 bit. Now I wanna generate 64-bit hashCode for user given Input.. Input may be string or Integer.
Please let me know.. take me out from this problem..
MY code follows like this...
package hash_table; public class Hash_table { private int num; private String data; public boolean equals(Object obj) { if(this == obj)
I have a simple classes here one is interface and another one is abstract class when i try to compile them abstract class is givving compilation error.
public interface MyInterface{ public void getName(); public void getName(String s); } public class HelloWorld{} abstract class SampleClass{
I have to meet certain requirements for this one program, and one of them is as follow: The program will require the user to enter the needed values one after another. If any of the entered values is invalid, then the program will detect that and go into a loop requiring the user to enter a valid value. The program must keep track of the total number of invalid values that the user has entered.
I know how to set a restriction and create a loop to keep asking the same question until it is satisfactory ( in this case I used a while loop) but I am at loss on how I should keep track of the total number of invalid values. I know I can use an increment but how do I set a variable for when the program meets an wrong input from the user.
Here is my code the whole program is working correctly but the Boolean equals and the has code and it is a requirement for the assignment. Why it is not working.
I know there are issues with the code I am new with java and was struggling so I have to clean my code up before I submit the assignment but for right now I have the out put the way I want it except the Boolean and hash code methods always output that the rectangles aren't equal even when I know they are and it outputs the not equal statement twice??
I am trying to implement the following example to override the equality and hashCode method if the class has reference type member. I do get the expected result "true" for equal and "false" for non-equal objects. But the print statement in the Circle's equal method is not executed when the objects values are not equal. I don't know what i am missing, though i get the equality result "false" as expected for non equal objects.
class Point{ private int x, y; Point (int x, int y) { this.x =x; this.y = y;
I'm trying to count the number of elements in an ArrayList which also have duplicates. So for example, in an ArrayList of strings which contains cat, cat, dog, horse, zebra, zebra, the answer should be two.
If an element is found to be a duplicate, that element should then be exempt from the search so if that element is found again it should not increase the duplicate count.
Here is my code:
public int countDuplicates() { int duplicates = 0; // TODO: Write the code to get the number of duplicates in the list for (int i = 0; i < list.size()-1;i++) { boolean found = false;
[Code] ....
I know it's wrong because right now it's still increasing the duplicate count for elements that have already been detected as duplicates. How can I make it stop doing this?
Write a method compressDuplicates that accepts a stack of integers as a parameter and that replaces each sequence of duplicates with a pair of values: a count of the number of duplicates, followed by the actual duplicated number. For example, suppose a variable called s stores the following sequence of values:
This new stack indicates that the original had 5 occurrences of 2 at the bottom of the stack followed by 2 occurrences of -5 followed by 4 occurrences of 3, and so on. This process works best when there are many duplicates in a row. For example, if the stack instead had stored:
bottom [10, 20, 10, 20, 20, 10] top
Then the resulting stack after the call ends up being longer than the original:
bottom [1, 10, 1, 20, 1, 10, 2, 20, 1, 10] top
If the stack is empty, your method should not change it. You may use one queue as auxiliary storage to solve this problem. You may not use any other auxiliary data structures to solve this problem, although you can have as many simple variables as you like. You may not use recursion to solve this problem. For full credit your code must run in O(n) time where n is the number of elements of the original stack.
I wrote a code but still having a problem with it , am I allowed to use 3 while loops ?
public void compressDuplicates(Stack<Integer> s ){ Stack<Integer> backup= new Stack<Integer>(); int count = 1; while(!s.isEmpty()){ int temp = s.pop();
I'm struggling with that piece of code, my intention is to check for the object I want to add before adding it, so there won't be any duplicate on my list. I'm not sure how could I do that, since I'm working with objects.
Person is a class with few parameters such as id, name, and few others.
I guess I should search for a person with the same id, since that has be unique, but can't get it right.
private ArrayList<person> model= new ArrayList<>(); //... if (model.contains(person))throw new IllegalArgumentException("duplicate"); else model.addElement(person);
It's supposed to count all of the duplicates in an array and print out how many occurrences of the value starting at whatever index, or if there are no duplicates state that. Basically:
No duplicates with value 1 beyond Index 0
There are 3 more occurrences of value 2 starting at index 1
There are 2 more occurrences of value 2 starting at index 2....
This is what I've got so far:
Java Code:
public static void main(String[] args) { int[] arr = {1, 2, 2, 3, 4, 2, 4, 3, 0, 5, 3, 2}; for(int i = 0; i<arr.length; i++){ int count = 0; for(int j = i+1; j<arr.length; j++){ if((arr[j] == arr[i]) && (i!=j)){ count++; System.out.print("There are " + count + " more occurrences of "); System.out.println(arr[i] + " starting at index " + i); } } } } mh_sh_highlight_all('java');
I am stuck on this exercise and I don't know what exactly is wrong. I think it's something with the .remove and the for each loop, but I am not sure.
public class seven { public static void main(String[] args) { ArrayList<String> list = new ArrayList<String>(); list.add("aaa"); list.add("brr"); list.add("unni");
[Code] ....
This is what i get
Exception in thread "main" java.util.ConcurrentModificationException at java.util.ArrayList$Itr.checkForComodification(Unknown Source) at java.util.ArrayList$Itr.next(Unknown Source) at seven.removeDuplicates(seven.java:24) at seven.main(seven.java:18)
In this array so many values are duplicates means 33 comes twice & 5 also comes twice & 9 comes three times. But I want to count the first value which is duplicate means 33 is first value which comes twice so answer would be 2.
I try:
public class FindFirstDuplicate { public static void main(String[] args) { int c=0; int[] a = {33,33,5,5,9,8,9,9}; outerloop: for(int i = 0; i < a.length; i++) {
