java.lang.ClassCastException: class java.lang.Integer cannot be cast to class java.lang.Long

Below example, the jdbcTemplate.queryForList returns an object of Integer and we try to convert it into a Long directly:

	public List<Customer> findAll() {
        String sql = "SELECT * FROM CUSTOMER";
        List<Customer> customers = new ArrayList<>();
        List<Map<String, Object>> rows = jdbcTemplate.queryForList(sql);
        for (Map row : rows) {
            Customer obj = new Customer();
            obj.setID(((Long) row.get("ID"))); // the object is an Integer
            obj.setName((String) row.get("NAME"));
            customers.add(obj);
        return customers;

Output

Caused by: java.lang.ClassCastException: class java.lang.Integer cannot be cast to class java.lang.Long 
(java.lang.Integer and java.lang.Long are in module java.base of loader 'bootstrap')
	at com.mkyong.misc.CustomerRepository.findAll(CustomerRepository.java:73)
	at com.mkyong.misc.CustomerRepository$$FastClassBySpringCGLIB$$7fc6ff36.invoke(<generated>)
	at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:218)
	at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.invokeJoinpoint(CglibAopProxy.java:749)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:163)
	at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:139)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186)
	at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:688)
	at com.mkyong.misc.CustomerRepository$$EnhancerBySpringCGLIB$$f96f7027.findAll(<generated>)
	at com.mkyong.StartApplication.startCustomerApp(StartApplication.java:103)
	at com.mkyong.StartApplication.run(StartApplication.java:72)
	at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:813)

Solution

To solve it, convert it back to the original Integer and cast it to Long

	obj.setID(((Integer) row.get("ID")).longValue());
	//obj.setID(((Long) row.get("ID")));

Java basic:

	Integer num = 1;
	Long numInLong = num.longValue();	 // Integer to Long
	Long numInLong2 = Long.valueOf(num); // Integer to Long