How to increase an array s length

Question

I have a quick question  I have an integer array in java that needs its length to vary throughout the class  Specifically  I need it to increase by one in certain situations  I tried  it like this           sbgHeadX   new int  numberOfSBG      I would increase the integer variable numberOfSBG when I needed to  but I don t think this works  Is there any other way

User · Answer

By definition arrays are fixed size  You can use instead an Arraylist wich is that  a  dynamic size  array  Actually what happens is that the VM  adjust the size   of the array exposed by the ArrayList   See also   using back-copy arrays

User · Answer

You can t increase the array s length if it is not declared in heap memory  see below code which first array input is asked by user and then it asks how much you want to increase array and also copy previous array elements    include lt stdio h gt   include lt stdlib h gt   int   increasesize int   p int   q int x        int i      for i 0 i lt x i                   q i  p i             free p       p q      return p    void display int   q int x        int i      for i 0 i lt x i                  printf  quot  d  n quot  q i                      int main         int x i      printf  quot enter no of element to create array quot        scanf  quot  d quot   amp x       int   p  int   malloc x sizeof int        printf  quot  n enter number in the array n quot        for i 0 i lt x i                  scanf  quot  d quot   amp p i              int y      printf  quot  nenter the new size to create new size of array quot        scanf  quot  d quot   amp y       int   q  int   malloc y sizeof int        display increasesize p q x  y       free q

User · Answer

First things first    In Java  once an array is created  it s length is fixed  Arrays cannot be resized   You can copy the elements of an array to a new array with a different size  The easiest way to do this  is to use one of the Arrays copyOf   methods  If you need a collection of variable size  you re probably better off using an ArrayList instead of an array    That being said  there might be situations where you have no other choice than to change the size of an array that is created somewhere outside of your code 1 The only way to do that is to manipulate the generated bytecode of the code that creates the array   Proof-of-concept  Below is a small proof-of-concept project that uses Java instrumentation to dynamically change the size of an array2  The sample project is a maven project with the following structure      - pom xml  - src     - main        - java           - com              - stackoverflow                 - agent                    - Agent java                    - test                       - Main java   Main java  This file contains the target class of which we re going to manipulate the bytecode   package com stackoverflow agent test   import java util Arrays   public class Main       public static void main String   args            String   array     Zero             fun array            System out println Arrays toString array               public static void fun String   array            array 1     One           array 2     Two           array 3     Three           array 4     Four             In the main method  we create a String array of size 1  In the fun method  4 additional values are assigned outside of the array s bounds  Running this code as-is will obviously result in an error   Agent java  This file contains the class that will perform the bytecode manipulation   package com stackoverflow agent   import java lang instrument ClassFileTransformer  import java lang instrument Instrumentation  import java security ProtectionDomain   public class Agent       public static void premain String args  Instrumentation instrumentation            instrumentation addTransformer new ClassFileTransformer                 public byte   transform ClassLoader l  String name  Class lt   gt  c                      ProtectionDomain d  byte   b                     if  name equals  com stackoverflow agent test Main                          byte iconst1    byte  0x04                      byte iconst5    byte  0x08                      byte anewarray    byte  0xbd                       for  int i   0  i  lt   b length - 1  i                              if  b i     iconst1  amp  amp  b i   1     anewarray                                b i    iconst5                                                                       return b                                     return null                                      On the bytecode level  the creation of the String array in the Main class consists of two commands    iconst 1  which pushes an int constant with value 1 onto the stack  0x04   anewarray  which pops the value of the stack and creates a reference array3 of the same size  0xbd   The above code looks for that combination of commands in the Main class  and if found  replaces the const 1 command with a const 5 command  0x08   effectively changing the dimensions of the array to 5 4     pom xml  The maven POM file is used to build the application JAR and configure the main class and the Java agent class 5   lt  xml version  1 0  encoding  UTF-8   gt   lt project xmlns  http   maven apache org POM 4 0 0           xmlns xsi  http   www w3 org 2001 XMLSchema-instance           xsi schemaLocation  http   maven apache org POM 4 0 0                              http   maven apache org xsd maven-4 0 0 xsd  gt     lt modelVersion gt 4 0 0 lt  modelVersion gt      lt groupId gt com stackoverflow lt  groupId gt     lt artifactId gt agent lt  artifactId gt     lt version gt 1 0-SNAPSHOT lt  version gt      lt build gt       lt finalName gt   project artifactId  lt  finalName gt       lt plugins gt         lt plugin gt           lt groupId gt org apache maven plugins lt  groupId gt           lt artifactId gt maven-jar-plugin lt  artifactId gt           lt version gt 3 1 1 lt  version gt           lt configuration gt             lt archive gt               lt manifestEntries gt                 lt Main-Class gt com stackoverflow agent test Main lt  Main-Class gt                 lt Premain-Class gt com stackoverflow agent Agent lt  Premain-Class gt                 lt Agent-Class gt com stackoverflow agent Agent lt  Agent-Class gt                 lt Can-Retransform-Classes gt true lt  Can-Retransform-Classes gt               lt  manifestEntries gt             lt  archive gt           lt  configuration gt         lt  plugin gt       lt  plugins gt     lt  build gt   lt  project gt    Build and execute  The sample project can be built using the standard mvn clean package command   Executing without referencing the agent code will yield the expected error     gt  java -jar target agent jar  Exception in thread  main  java lang ArrayIndexOutOfBoundsException  1         at com stackoverflow agent test Main fun Main java 15          at com stackoverflow agent test Main main Main java 9    While executing with the agent code will yield     gt  java -javaagent target agent jar -jar target agent jar  Zero  One  Two  Three  Four    This demonstrates that the size of the array was successfully changed using bytecode manipulation     1 Such situations came up in questions here and here  the latter of which prompted me to write this answer  2 Technically  the sample project doesn t resize the array  It just creates it with a different size than the size specified in code  Actually resizing an existing array while maintaining its reference and copying its elements would be a fair bit more complicated  3 For a primitive array  the corresponding bytecode operation would be newarray  0xbc  instead  4 As noted  this is just a proof of concept  and a very hacky one at that   Instead of randomly replacing bytes  a more robust implementation could use a bytecode manipulation library like ASM to insert a pop command followed by an sipush command before any newarray or anewarray command  Some more hints towards that solution can be found in the comments to this answer  5 In a real-world scenario  the agent code would obviously be in a separate project

User · Answer

I would suggest you use an ArrayList as you won t have to worry about the length anymore  Once created  you can t modify an array size      An array is a container object that holds a fixed number of values of a single type  The length of an array is established when the array is created  After creation  its length is fixed     Source

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If you don t want or cannot use ArrayList  then there is a utility method   Arrays copyOf      that will allow you to specify new size  while preserving the elements

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Example of Array length change method  with old data coping    static int   arrayLengthChange int   arr  int newLength        int   arrNew   new int newLength       System arraycopy arr  0  arrNew  0  arr length       return arrNew

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You can make use of ArrayList  Array has the fixed number of size    This Example here can help you  The example is pretty easy with its output     Output  2 5 1 23 14  New length  20  Element at Index 5 29  List size  6  Removing element at index 2  1  2 5 23 14 29

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Arrays in Java are of fixed size that is specified when they are declared  To increase the size of the array you have to create a new array with a larger size and copy all of the old values into the new array   ex   char   copyFrom       a    b    c    d    e     char   copyTo      new char 7    System out println Arrays toString copyFrom    System arraycopy copyFrom  0  copyTo  0  copyFrom length   System out println Arrays toString copyTo      Alternatively you could use a dynamic data structure like a List

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Item   newItemList   new  Item itemList length 1         for loop to go thorough the list one by one     for int i 0  i lt  itemList length i               value is stored here in the new list from the old one         newItemList i  itemList i               all the values of the itemLists are stored in a bigger array named newItemList     itemList newItemList

[java] How to increase an array's length

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