map 空指针异常

java.util..get( key)

• get get( key)
  Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

  More formally, if this map contains a mapping from a key k to a value v such that (key==null ? k==null : key.equals(k)), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)

  If this map permits null values, then a return value of null does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to null. The operation may be used to distinguish these two cases.

  Parameters: key - the key whose associated value is to be returned Returns: the value to which the specified key is mapped, or null if this map contains no mapping for the key Throws: - if the key is of an inappropriate type for this map () - if the specified key is null and this map does not permit null keys ()

表结构如下：

create table user(

id int not null primary key,

roleid int not null ,

deptid int not null,

tlid int null,

email varchar(50) not null,

password varchar(50) not null
);

当使用map的get方法时抛出null异常，

if (map.get("tlid")!=null) {
                user.setTlid((Integer)map.get("tlid"));
            }

将代码改为以下：显示正常。

if(map.containsKey("tlid")){
            //if (!map.get("tlid").equals(null)) {
            if (map.get("tlid")!=null) {
                user.setTlid((Integer)map.get("tlid"));
            }

        }

以下两句代码的区别：

if (!map.get("tlid").equals(null)) {

当map.get("tlid")为null时，抛出空指针异常。正确写法：

if (map.get("tlid")!=null) {

  完整代码：       

private User populate(Map<String, Object> map) {
        User user = new User();
        user.setId((Integer) map.get("id"));
        user.setRoleid((Integer) map.get("roleid"));
        user.setDeptid((Integer)map.get("deptid"));
        /*
         * foreign key..,no map.. omit these attr
         */
        if(map.containsKey("tlid")){
            //if (!map.get("tlid").equals(null)) {
            if (map.get("tlid")!=null) {
                user.setTlid((Integer)map.get("tlid"));
            }

        }
        user.setEmail((String) map.get("email"));
        user.setPassword((String) map.get("password"));

        return user;
    }