autoboxing

问题 Can someone explain to me the usage of Integer, Boolean etc in place of their primitive types in JAVA? I can't seem to grasp the advantages their are providing. They seem to create unnecessary problems of handling null values. Thanks! 回答1: Boolean , Integer , Long , ... are Objects. You can use them in places where you can't use primitive types, e.g. storing them in a Collection like a Map using them as template parameter assigning them a null value using them in a more general way (e.g. Long

问题 I recently stumbled on a question that made me stop and think... To me, the code below should always trigger an error, but when one of my colleagues asked me why Eclipse didn't show one, I couldn't answer anything. class A { public static void main(String... args) { System.out.println(new Object() == 0); } } I've investigated and found that with source level 1.6 it indeed throws an error: incomparable types: Object and int But now in 1.7 it compiles ok. Please, what new feature does warrant

问题 We know that by default, the 'obj' below is string. Without using 'parseInt', how does JavaScript compare it with a number? obj = document.frm.weight.value; if( obj < 0 || obj > 5 ){ alert("Enter valid range!"); return false; } 回答1: If one of the operands of < or > is number, the other one will be casted to a number. alert("3" > 3); // false alert("3.5" > 3); // true EDIT and further explanation: If it is not possible to cast the other parameter into a number, it is casted into the special

问题 I'm trying to understand the behavior of Scala for-loop implicit box/unboxing of "numerical" types. Why does the two first fail but not the rest? 1) Fails: scala> for (i:Long <- 0 to 10000000L) {} <console>:19: error: type mismatch;<br> found : Long(10000000L) required: Int for (i:Long <- 0 to 10000000L) {} ^ 2> Fails: scala> for (i <- 0 to 10000000L) {} <console>:19: error: type mismatch; found : Long(10000000L) required: Int for (i <- 0 to 10000000L) {} ^ 3) Works: scala> for (i:Long <- 0L

问题 It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center. Closed 7 years ago . I program in Java, C and Python. The rule for automatic coercions among arithmetic types have been augmented to handle the richer set of types Source: "The C Programming Language" But what does "coercion" mean?

问题 This question already has answers here : What is “compiler compliance level” in Eclipse? (4 answers) Closed 3 years ago . I am trying to implement autoboxing but it's showing an error: Cannot convert from int to integer. package JavaTraining; public class Wrap { public static void main(String ar[]) { Integer a=100; String i=a.toString(); System.out.println(i); System.out.println(a); } } Eclispe version is 3.1 回答1: (This is relevant to Eclipse specifically) - You need to change your Compiler

问题 I asked: Is autoboxing/unboxing is done at runtime (JVM) or compile time (compiler)? I received this answer: Autoboxing is achieved by the insertion of method calls and casts by the compiler, into the code. These calls and casts are handled at runtime. Please explain in more detail. 回答1: From Java Specification Chapter 5. Conversions and Promotions Every expression written in the Java programming language has a type that can be deduced from the structure of the expression and the types of the

问题 I have a method that returns a Long object datatype via invocation of: resp.getResultCode() . I want to compare it HttpStatus.GONE.value() which actually just returns a primitive int value of 410 . Would the Long unbox itself to properly compare with the int primitive? if(resp.getResultCode() == HttpStatus.GONE.value()){ // code inside.. } 回答1: Here's the JLS explanation If the operands of an equality operator are both of numeric type, or one is of numeric type and the other is convertible (

问题 So I understand you can have object streams, i.e. Stream<T> and specialist primitive streams, e.g. IntStream , DoubleStream , etc. One of the benefits of the latter is to avoid autoboxing. Also if we take IntStream as an example, it has specialised operations such as filter that accepts an IntPredicate . I wondered if I had an IntStream vs a Stream<Integer> at which point you're saving on boxing, e.g. intstream.filter(x -> x >= 50).forEach(x -> System.out.println(x)); vs stream.filter(x -> x

问题 I am new to java . I am now learning the non-primitive Integer type in java . I know the following comparison is not valid and throws a compilation error - String str = "c"; Char chr = 'c'; if(str == chr) return true; The above code snippet gives me the - "Test.java:lineNumber: incomparable types: java.lang.String and char" errors. But I found the following code snippet compiles fine - int a = 1234; Integer aI = 1234; if(a==aI) return true; Here, a is primitive int and aI is non-primitive. So