Android Java 自定义错误

package com;public class ZeroException extends Exception {    private static final long serialVersionUID = 1L;    public ZeroException(){        super() ;    }        public ZeroException ( String string ) {        super( string ) ;    }}

package com;public class Test {    public static void main(String[] args) {        Test test = new Test() ;                try {            test.fun( "" ) ;        } catch (ZeroException e) {            e.printStackTrace();        }     }    void fun( String string ) throws ZeroException {        if( string == null ){            throw new ZeroException( "参数不能为null" ) ;        }        if( string == "" ) {            throw new ZeroException( "参数不能为空" ) ;        }    }}

com.ZeroException: 参数不能为空    at com.Test.fun(Test.java:22)    at com.Test.main(Test.java:9)