blocks|key|1316888|text|编译器正在抱怨，因为您当前在for循环中有'if(exist)‘块。它需要在它之外。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1316889|for(int+i=0;i<this.customers.size();i%2B%2B){
++++++++if(this.customers.get(i).getId()+==+id){
++++++++++++exist=true;
++++++++++++break;
++++++++}
}

if(exist)+{
++++return+this.customers.get(id);
}+else+{
++++return+this.customers.get(id);
}|code-block|syntax|javascript|1316890|也就是说，有更好的方法来执行此搜索。就我个人而言，如果我使用的是ArrayList，我的解决方案将与Jon+Skeet发布的解决方案类似。|1316891|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|$]]

The compiler is complaining because you currently have the 'if(exist)' block inside of your for loop. It needs to be outside of it.

<pre><code>for(int i=0;i&lt;this.customers.size();i++){
 if(this.customers.get(i).getId() == id){
 exist=true;
 break;
 }
}

if(exist) {
 return this.customers.get(id);
} else {
 return this.customers.get(id);
}
</code></pre>

That being said, there are better ways to perform this search. Personally, if I were using an ArrayList, my solution would look like the one that Jon Skeet has posted.

blocks|key|1532072|text|Customer+findCustomerByid(int+id){
++++for+(int+i=0;+i<this.customers.size();+i%2B%2B)+{
++++++++Customer+customer+=+this.customers.get(i);
++++++++if+(customer.getId()+==+id){
+++++++++++++return+customer;
++++++++}
++++}
++++return+null;+//+no+Customer+found+with+this+ID;+maybe+throw+an+exception
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1532073|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>Customer findCustomerByid(int id){
 for (int i=0; i&lt;this.customers.size(); i++) {
 Customer customer = this.customers.get(i);
 if (customer.getId() == id){
 return customer;
 }
 }
 return null; // no Customer found with this ID; maybe throw an exception
}
</code></pre>

blocks|key|1532155|text|您遗漏了return语句，因为如果列表大小为0，则for循环将永远不会执行，因此if将永远不会运行，因此您永远不会返回。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1532156|将if语句移出循环。|1532157|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

You're missing the return statement because if your list size is 0, the for loop will never execute, thus the if will never run, and thus you will never return.

Move the if statement out of the loop.

blocks|key|1532236|text|就我个人而言，我现在很少自己编写循环，当我可以逃脱惩罚的时候……我使用Jakarta+commons库：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1532237|Customer+findCustomerByid(final+int+id){
++++return+(Customer)+CollectionUtils.find(customers,+new+Predicate()+{
++++++++public+boolean+evaluate(Object+arg0)+{
++++++++++++return+((Customer)+arg0).getId()==id;
++++++++}
++++});
}|code-block|syntax|javascript|1532238|耶！我只保留了一行！|1532239|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|$]]

Personally I rarely write loops myself now when I can get away with it... I use the Jakarta commons libs:

<pre><code>Customer findCustomerByid(final int id){
 return (Customer) CollectionUtils.find(customers, new Predicate() {
 public boolean evaluate(Object arg0) {
 return ((Customer) arg0).getId()==id;
 }
 });
}
</code></pre>

Yay! I saved one line!

blocks|key|1317145|text|即使这个话题已经很老了，我还是想补充一点。如果您为您的类覆盖equals，以便它比较您的getId，您可以使用：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1317146|customer+=+new+Customer(id);
customers.get(customers.indexOf(customer));|code-block|syntax|javascript|1317147|当然，您必须检查IndexOutOfBounds-Exception，它可能会被转换为空指针或自定义CustomerNotFoundException。|1317148|entityMap^0|U|6|18|5|0|0|8|G|1E|P|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|Z|8|@]|D|@]|E|$]]]|N|$]]

Even if that topic is quite old, I'd like to add something.
If you overwrite <code>equals</code> for you classes, so it compares your <code>getId</code>, you can use:

<pre><code>customer = new Customer(id);
customers.get(customers.indexOf(customer));
</code></pre>

Of course, you'd have to check for an <code>IndexOutOfBounds</code>-Exception, which oculd be translated into a null pointer or a custom <code>CustomerNotFoundException</code>.

blocks|key|1317213|text|我做了类似的事情，编译器发现您的return语句在If()语句中。如果希望解决此错误，只需在If语句之前创建一个名为customerId的新局部变量，然后在If语句内赋值。在if语句之后，调用return语句，并返回cstomerId。如下所示：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1317214|Customer+findCustomerByid(int+id)
{
++++boolean+exist=false;

++++if(this.customers.isEmpty())+{
++++++++return+null;
++++}

++++for(int+i=0;i<this.customers.size();i%2B%2B)+{
++++++++if(this.customers.get(i).getId()+==+id)+{
++++++++++++exist=true;
++++++++++++break;
++++++++}

++++++++int+customerId;

++++++++if(exist)+{
++++++++++++customerId+=+this.customers.get(id);
++++++++}+else+{
++++++++++++customerId+=+this.customers.get(id);
++++++++}
++++}
++++return+customerId;
}|code-block|syntax|javascript|1317215|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|$]]

I did something close to that, the compiler is seeing that your return statement is in an If() statement. If you wish to resolve this error, simply create a new local variable called customerId before the If statement, then assign a value inside of the if statement. After the if statement, call your return statement, and return cstomerId.
Like this:

<pre><code>Customer findCustomerByid(int id)
{
 boolean exist=false;

 if(this.customers.isEmpty()) {
 return null;
 }

 for(int i=0;i&lt;this.customers.size();i++) {
 if(this.customers.get(i).getId() == id) {
 exist=true;
 break;
 }

 int customerId;

 if(exist) {
 customerId = this.customers.get(id);
 } else {
 customerId = this.customers.get(id);
 }
 }
 return customerId;
}
</code></pre>

blocks|key|1532418|text|在Java+8中：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1532419|Customer+findCustomerByid(int+id)+{
++++return+this.customers.stream()
++++++++.filter(customer+->+customer.getId().equals(id))
++++++++.findFirst().get();
}|code-block|syntax|javascript|1532420|将返回类型更改为Optional<Customer>也可能更好。|offset|length|style|CODE|1532421|entityMap^0|0|0|8|I|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]]|$1|G|3|H|5|6|7|Q|8|@$I|R|J|S|K|L]]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

In Java 8:

<pre><code>Customer findCustomerByid(int id) {
 return this.customers.stream()
 .filter(customer -&gt; customer.getId().equals(id))
 .findFirst().get();
}
</code></pre>

It might also be better to change the return type to <code>Optional&lt;Customer&gt;</code>.

I'm trying to figure out the best way to search a customer in an <code>ArrayList</code> by its Id number. The code below is not working; the compiler tells me that I am missing a <code>return</code> statement.

<pre><code>Customer findCustomerByid(int id){
 boolean exist=false;

 if(this.customers.isEmpty()) {
 return null;
 }

 for(int i=0;i&lt;this.customers.size();i++) {
 if(this.customers.get(i).getId() == id) {
 exist=true;
 break;
 }

 if(exist) {
 return this.customers.get(id);
 } else {
 return this.customers.get(id);
 }
 }

}

//the customer class is something like that
public class Customer {
 //attributes
 int id;
 int tel;
 String fname;
 String lname;
 String resgistrationDate;
}
</code></pre>

search in java ArrayList

编译器

Java

我正在尝试找出在ArrayList中通过Id号搜索客户的最佳方法。下面的代码不起作用；编译器告诉我缺少一条return语句。Customer findCustomerByid(int id){    boolean exist=false;    if(this.customers.isEmpty()) {      ...

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