java : cannot set Map field

Question

I have a simple bean like this:

class Account {
    private String username;
    private String password;
    private Map<String, String> extras;

    String getUsername() {
        return username;
    }

    void setUsername(String username) {
        this.username = username;
    }

    String getPassword() {
        return password;
    }

    void setPassword(String password) {
        this.password = password;
    }

    Map<String, String> getExtras() {
        return extras;
    }

    void setExtras(Map<String,String> attr) {
        this.extras=attr;
    }
 }

now I'm going to set extra by:

 Account tmpAccount=new Account();
 tmpAccount.setExtras(new HashMap<String, String>().put("x","y"));

but I got this error:

 setExtras(Map<String,String> in Account cannot be applied to Object.

Why?

Because new HashMap<String, String>().put("x","y") doesn't bring you the same HashMap, the method put("x","y") will rather bring you the last object mapped as "x" — morgano
– morgano, Commented Jul 14, 2014 at 5:05
To avoid ugly nullPointException change to private Map<String, String> extras = Collections.emptyMap(); and set your Strings to "" — javajon
– javajon, Commented Jul 14, 2014 at 5:12

Elliott Frisch · Accepted Answer · 2014-07-14 05:06:44Z

3

If I understand your question, the issue is you can't chain the HashMap put,

Map<String, String> map = new HashMap<>();
tmpAccount.setExtras(map.put("x","y"));

Per the Map#put() method defintion,

V put(K key, V value)

It returns V, not Map. And setExtras(map); takes a Map (not a String).

answered Jul 14, 2014 at 5:06

Elliott Frisch

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Comments

Jigar Joshi · Accepted Answer · 2014-07-14 05:16:09Z

2

new HashMap<String, String>().put("x","y")

this statement returns a String instance

void setExtras(Map<String,String> attr)

See put(K, V) API doc for reference

edited Jul 14, 2014 at 5:16

answered Jul 14, 2014 at 5:06

Jigar Joshi

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1 Comment

David Conrad Over a year ago

It will not return "y" but the previous mapping for "x" and since there wasn't any, it will return null.

Nikhil Talreja · Accepted Answer · 2014-07-14 05:19:11Z

0

.put() returns the previous value associated with key, or null if there was no mapping for key. You should do something like this:

Account tmpAccount=new Account();
Map<String, String> values = new HashMap<String, String>();
values.put("x","y");
tmpAccount.setExtras(values);

edited Jul 14, 2014 at 5:19

answered Jul 14, 2014 at 5:08

Nikhil Talreja

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1 Comment

David Conrad Over a year ago

No, put() returns the previous mapping for the key inserted, if any, or null if there is none.

Super Hornet · Accepted Answer · 2014-07-14 05:20:57Z

0

There is another solution that I just found, casting to Map:

  tmpAccount.setExtras(Map<String, String>)new HashMap<String, String>().put("x","y"));

It works properly.

answered Jul 14, 2014 at 5:20

Super Hornet

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Comments

vinayknl · Accepted Answer · 2014-07-14 05:23:47Z

0

Account tmpAccount=new Account();
tmpAccount.setExtras(new HashMap<String, String>().put("x","y"));

will not work. Use as below

  Account tmpAccount=new Account();
  Map<String, String> myExtras = new HashMap<String, String>();
  myExtras.put("x","y");
  tmpAccount.setExtras(myExtras);

Reason, HashMap's put method signature is as below

 public V put(K key,V value)

and the value it returns is

Returns:
        the previous value associated with key, or null if there was no mapping for key. (A null return can      
         also indicate that the map previously associated null with key.)

So, when you do

tmpAccount.setExtras(new HashMap<String, String>().put("x","y"));

You are setting null to setExtras where as it is expecting a HashMap

edited Jul 14, 2014 at 5:23

answered Jul 14, 2014 at 5:07

vinayknl

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Muhammad · Accepted Answer · 2014-07-14 05:27:23Z

0

The Method put() returns a string object while the method setExtras(Map m) is expecting an object of calls Map, Try the following :)

    Account tmpAccount = new Account();
    Map map = new HashMap<String, String>();
    map.put("x", "y");
    tmpAccount.setExtras(map);

answered Jul 14, 2014 at 5:27

Muhammad

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