blocks|key|319717|text|方法arrayList.size()+returns列表中的项数-所以如果索引大于或等于size()，它就不存在。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|319718|if(index+>=+myList.size()){
++//index+not+exists
}else{
+//+index+exists
}|code-block|syntax|javascript|319719|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/7/docs/api/java/util/Hashtable.html#size()^0|2|G|18|6|J|7|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@$9|X|A|Y|1|Z]]|E|$]]|$1|F|3|G|5|H|7|10|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|11|8|@]|D|@]|E|$]]]|L|$M|$5|N|O|P|E|$Q|R]]]]

The method <code>arrayList.size()</code> <a href="http://docs.oracle.com/javase/7/docs/api/java/util/Hashtable.html#size()" rel="noreferrer">returns</a> the number of items in the list - so if the index is greater than or equal to the <code>size()</code>, it doesn't exist.

<pre><code>if(index &gt;= myList.size()){
 //index not exists
}else{
 // index exists
}
</code></pre>

blocks|key|319721|text|如果您的索引小于列表的大小，则它确实存在，可能带有null值。如果索引更大，那么您可以调用ensureCapacity()来使用该索引。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|319722|如果要检查索引处的值是否为null，请调用get()|319723|entityMap^0|P|4|19|G|0|D|4|L|5|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|I|$]]

If your index is less than the size of your list then it does exist, possibly with <code>null</code> value. If index is bigger then you may call <code>ensureCapacity()</code> to be able to use that index.

If you want to check if a value at your index is <code>null</code> or not, call <code>get()</code>

blocks|key|3279600|text|您可以使用size()方法检查ArrayList的大小。这将返回最大索引%2B1|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3279601|entityMap^0|5|6|F|9|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

You can check the size of an <code>ArrayList</code> using the <code>size()</code> method. This will return the maximum index +1

blocks|key|321737|text|您可以检查数组的大小。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|321738|package+sojava;
import+java.util.ArrayList;

public+class+Main+{
++++public+static+Object+get(ArrayList+list,+int+index)+{
++++++++if+(list.size()+>+index)+{+return+list.get(index);+}
++++++++return+null;
++++}

++++public+static+void+main(String[]+args)+{
++++++++ArrayList+list+=+new+ArrayList();
++++++++list.add("");+list.add("");+list.add("");++++++++
++++++++System.out.println(get(list,+4));
++++++++//+prints+'null'
++++}
}|code-block|syntax|javascript|321739|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You could check for the size of the array.

<pre><code>package sojava;
import java.util.ArrayList;

public class Main {
 public static Object get(ArrayList list, int index) {
 if (list.size() &gt; index) { return list.get(index); }
 return null;
 }

 public static void main(String[] args) {
 ArrayList list = new ArrayList();
 list.add(""); list.add(""); list.add(""); 
 System.out.println(get(list, 4));
 // prints 'null'
 }
}
</code></pre>

blocks|key|3066614|text|这就是你需要的..。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3066615|public+boolean+indexExists(final+List+list,+final+int+index)+{
++++return+index+>=+0+&&+index+<+list.size();
}|code-block|syntax|javascript|3066616|为什么不使用普通的旧数组呢？我认为对列表的索引访问是一种代码味道。|3066617|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

This is what you need ...

<pre><code>public boolean indexExists(final List list, final int index) {
 return index &gt;= 0 &amp;&amp; index &lt; list.size();
}
</code></pre>

Why not use an plain old array? Indexed access to a List is a code smell I think.

blocks|key|318861|text|关于你的更新(这可能是另一个问题)。您应该使用这些对象的数组，而不是ArrayList，这样您就可以简单地检查空值：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|318862|Object[]+array+=+new+Object[MAX_ENTRIES];
..
if+(+array[+8+]+==+null+)+{
+++//+not+available
}
else+{
+++//+do+something
}|code-block|syntax|javascript|318863|最佳实践|318864|如果你的数组中没有上百个条目，你应该考虑把它组织成一个类，去掉3,8等魔术数字。|318865|使用异常的控制流是不好的做法。|318866|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

Regarding your update (which probably should be another question).
You should use an array of these objects instead an ArrayList,
so you can simply check the value for null:

<pre><code>Object[] array = new Object[MAX_ENTRIES];
..
if ( array[ 8 ] == null ) {
 // not available
}
else {
 // do something
}
</code></pre>

Best-Practice

If you don't have hundred of entries in your array you should consider organizing it as a class to get rid of the magic numbers 3,8 etc.

Control flow using exception is bad practice.

blocks|key|319771|text|虽然你得到了十几个关于使用列表大小的建议，这些建议适用于具有线性条目的列表，但似乎没有人阅读你的问题。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|319772|如果您在不同的索引处手动添加条目，这些建议都不会起作用，因为您需要检查特定的索引。|319773|使用if+(+list.get(index)+==+null+)也不起作用，因为get()抛出一个异常，而不是返回null。|319774|试试这个：|319775|try+{
++++list.get(+index+);
}+catch+(+IndexOutOfBoundsException+e+)+{
++++list.add(+index,+new+Object()+);
}|code-block|syntax|javascript|319776|在这里，如果索引不存在，则添加一个新条目。你可以改变它来做一些不同的事情。|319777|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|S|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|T|8|@]|9|@]|A|$]]|$1|H|3|I|5|J|7|U|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|V|8|@]|9|@]|A|$]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

While you got a dozen suggestions about using the size of your list, which work for lists with linear entries, no one seemed to read your question.

If you add entries manually at different indexes none of these suggestions will work, as you need to check for a specific index.

Using if ( list.get(index) == null ) will not work either, as get() throws an exception instead of returning null.

Try this:

<pre><code>try {
 list.get( index );
} catch ( IndexOutOfBoundsException e ) {
 list.add( index, new Object() );
}
</code></pre>

Here a new entry is added if the index does not exist. You can alter it to do something different.

blocks|key|3066779|text|快速测试索引是否存在。在你的实现中，用你正在测试的list替换list。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3066780|public+boolean+hasIndex(int+index){
++++if(index+<+list.size())
++++++++return+true;
++++return+false;
}|code-block|syntax|javascript|3066781|或者对于二维ArrayLists。|3066782|public+boolean+hasRow(int+row){
++++if(row+<+_matrix.size())
++++++++return+true;
++++return+false;
}|3066783|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Quick and dirty test for whether an index exists or not. in your implementation replace list With your list you are testing.

<pre><code>public boolean hasIndex(int index){
 if(index &lt; list.size())
 return true;
 return false;
}
</code></pre>

or for 2Dimensional ArrayLists...

<pre><code>public boolean hasRow(int row){
 if(row &lt; _matrix.size())
 return true;
 return false;
}
</code></pre>

blocks|key|3066788|text|从Java9开始，有一种检查索引是否属于数组的标准方法--+Objects#checkIndex()：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3066789|List<Integer>+ints+=+List.of(1,2,3);
System.out.println(Objects.checkIndex(1,ints.size()));+//+1
System.out.println(Objects.checkIndex(10,ints.size()));+//IndexOutOfBoundsException|code-block|syntax|javascript|3066790|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.oracle.com/javase/9/docs/api/java/util/Objects.html#checkIndex-int-int-^0|U|K|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

Since Java 9 there is a standard way of checking if an index belongs to the array - <a href="https://docs.oracle.com/javase/9/docs/api/java/util/Objects.html#checkIndex-int-int-" rel="nofollow noreferrer">Objects#checkIndex()</a> :
<pre><code>List&lt;Integer&gt; ints = List.of(1,2,3);
System.out.println(Objects.checkIndex(1,ints.size())); // 1
System.out.println(Objects.checkIndex(10,ints.size())); //IndexOutOfBoundsException
</code></pre>

blocks|key|318914|text|通常我只检查索引是否小于数组大小|type|unstyled|depth|inlineStyleRanges|entityRanges|data|318915|if+(index+<+list.size())+{
++++...
}|code-block|syntax|javascript|318916|如果您还担心索引为负值，请使用以下命令|318917|if+(index+>=+0+&&+index+<+list.size())+{
++++...
}|318918|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Usually I just check if the index is less than the array size

<pre><code>if (index &lt; list.size()) {
 ...
}
</code></pre>

If you are also concerned of index being a negative value, use following

<pre><code>if (index &gt;= 0 &amp;&amp; index &lt; list.size()) {
 ...
}
</code></pre>

blocks|key|319846|text|一种简单的方法来完成此操作：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|319847|try+{
++list.get(+index+);+
}+
catch+(+IndexOutOfBoundsException+e+)+{
++if(list.isEmpty()+%7C%7C+index+>=+list.size()){
++++//+Adding+new+item+to+list.
++}
}|code-block|syntax|javascript|319848|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

a simple way to do this:

<pre><code>try {
 list.get( index ); 
} 
catch ( IndexOutOfBoundsException e ) {
 if(list.isEmpty() || index &gt;= list.size()){
 // Adding new item to list.
 }
}
</code></pre>

blocks|key|3066903|text|这是给Kotlin开发人员的，在这里结束：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3066904|if+(index+in+myList.indices)+{
++//+index+is+valid
}|code-block|syntax|javascript|3066905|其他解决方案：|3066906|if+(index+in+0..myArray.lastIndex)+{
++//+index+is+valid
}|3066907|if+(index+>=+0+&&+index+<=+myList.lastIndex)+{
++//+index+is+valid
}|3066908|//+Note:+elements+of+the+list+should+be+non-null
if+(myList.getOrNull(index)+!=+null)+{
++//+index+is+valid
}|3066909|//+Note:+elements+of+the+list+should+be+non-null
myList.getOrNull(index)?.let+{+element+->
++//+index+is+valid;+use+the+element
}|3066910|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|D|7|W|8|@]|9|@]|A|$E|F]]|$1|M|3|N|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|D|7|Y|8|@]|9|@]|A|$E|F]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

This is for Kotlin developers ending up here:
<pre class="lang-kotlin prettyprint-override"><code>if (index in myList.indices) {
 // index is valid
}
</code></pre>
Other solutions:
<pre class="lang-kotlin prettyprint-override"><code>if (index in 0..myArray.lastIndex) {
 // index is valid
}
</code></pre>
<pre class="lang-kotlin prettyprint-override"><code>if (index &gt;= 0 &amp;&amp; index &lt;= myList.lastIndex) {
 // index is valid
}
</code></pre>
<pre class="lang-kotlin prettyprint-override"><code>// Note: elements of the list should be non-null
if (myList.getOrNull(index) != null) {
 // index is valid
}
</code></pre>
<pre class="lang-kotlin prettyprint-override"><code>// Note: elements of the list should be non-null
myList.getOrNull(index)?.let { element -&gt;
 // index is valid; use the element
}
</code></pre>

I'm using <code>ArrayList&lt;String&gt;</code> and I add data at specific indices, how can I check if a specific index exists?
Should I simply <code>get()</code> and check the value? Or should I wait for an exception?
Is there another way?
Update: Thank you for your answers, but because I'm only adding stuff at specific indices, the length of the list will not show me which are available.

ArrayList - how can I check if an index exists?

Kotlin

我正在使用ArrayList<String>，并且我在特定的索引处添加数据，如何检查特定的索引是否存在？我是否应该简单地get()并检查该值？或者我应该等待一个异常？还有别的办法吗？更新：谢谢你的回答，但因为我只在特定的索引上添加内容，所以列表的长度不会显示哪些是可用的。

问ArrayList -如何检查索引是否存在？
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