Curly Braces in "New" Expression? (E.G. "New Myclass() { ... }")

Curly braces in new expression? (e.g. new MyClass() { ... })

This is the syntax for creating an instance of anonymous class that extends Handler. This is part of Java.

Code with curly braces in/after new?

The initialValue() method of ThreadLocal is just a way to construct a ThreadLocal holding a value other than null.

Edit: Oh, I see that's not what you're asking about. What you have there is the same as if you did:

public class MyOwnThreadLocal extends ThreadLocal {
    public Connection initialValue() {
        return DriverManager.getConnection(DB_URL);
    }
}

Except your version doesn't require a completely separate class definition--hence it's called an "anonymous class".

What are the advantages of list initialization (using curly braces)?

Basically copying and pasting from Bjarne Stroustrup's "The C++ Programming Language 4th Edition":

List initialization does not allow narrowing (§iso.8.5.4). That is:

• An integer cannot be converted to another integer that cannot hold its value. For example, char
  to int is allowed, but not int to char.
• A floating-point value cannot be converted to another floating-point type that cannot hold its
  value. For example, float to double is allowed, but not double to float.
• A floating-point value cannot be converted to an integer type.
• An integer value cannot be converted to a floating-point type.

Example:

void fun(double val, int val2) {

    int x2 = val;    // if val == 7.9, x2 becomes 7 (bad)

    char c2 = val2;  // if val2 == 1025, c2 becomes 1 (bad)

    int x3 {val};    // error: possible truncation (good)

    char c3 {val2};  // error: possible narrowing (good)

    char c4 {24};    // OK: 24 can be represented exactly as a char (good)

    char c5 {264};   // error (assuming 8-bit chars): 264 cannot be 
                     // represented as a char (good)

    int x4 {2.0};    // error: no double to int value conversion (good)

}

---

The only situation where = is preferred over {} is when using auto keyword to get the type determined by the initializer.

Example:

auto z1 {99};   // z1 is an int
auto z2 = {99}; // z2 is std::initializer_list<int>
auto z3 = 99;   // z3 is an int

---

Conclusion

Prefer {} initialization over alternatives unless you have a strong reason not to.

Java programming language syntax

The code following curly brackets is syntax for an anonymous class. This is similar to a lambda expression you may have seen in C++11, except that it defines an entire class versus just one function (so an anonymous class instead of an anonymous function).

java just curly braces

It's a code block. The variables declared in there are not visible in the upper block (method body outside of these curlies), i.e. they have a more limited scope.

---

Related Topics