其实很不一样:

声明发生在构建之前。因此，如果在两个位置都初始化了变量(本例中为b)，构造函数的初始化将取代在类级别所做的初始化。

所以在类级别声明变量，在构造函数中初始化它们。

我个人的“原则”(几乎从未被打破)是:

的开头声明所有变量 一块 使所有变量为final，除非它们 不能 每行声明一个变量 永远不要在某处初始化变量 宣布 只初始化a中的东西 构造函数时使用 的构造函数 初始化

我的代码是这样的:

public class X
{
    public static final int USED_AS_A_CASE_LABEL = 1; // only exception - the compiler makes me
    private static final int A;
    private final int b;
    private int c;

    static 
    { 
        A = 42; 
    }

    {
        b = 7;
    }

    public X(final int val)
    {
        c = val;
    }

    public void foo(final boolean f)
    {
        final int d;
        final int e;

        d = 7;

        // I will eat my own eyes before using ?: - personal taste.
        if(f)
        {
            e = 1;
        }
        else
        {
            e = 2;
        }
    }
}

This way I am always 100% certain where to look for variables declarations (at the start of a block), and their assignments (as soon as it makes sense after the declaration). This winds up potentially being more efficient as well since you never initialize a variable with a value that is not used (for example declare and init vars and then throw an exception before half of those vars needed to have a value). You also do not wind up doing pointless initialization (like int i = 0; and then later on, before "i" is used, do i = 5;.

我非常重视一致性，所以遵循这个“规则”是我一直在做的事情，它使处理代码变得更容易，因为你不需要到处寻找东西。

你的里程可能会有所不同。

我认为例2更可取。我认为最佳实践是在构造函数外部声明，并在构造函数中初始化。

第二种选择更可取，因为它允许在ctors中使用不同的逻辑进行类实例化，并使用ctors链接。如。

class A {
    int b;

    // secondary ctor
    A(String b) {
         this(Integer.valueOf(b));
    }

    // primary ctor
    A(int b) {
         this.b = b;
    }
}

所以第二种选择更加灵活。

    class MyClass extends FooClass {
    String a = null;

    public MyClass() {
        super();     // Superclass calls init();
    }

    @Override
    protected void init() {
        super.init();
        if (something)
            a = getStringYadaYada();
    }
}

关于以上，

String a = null;

Null init可以避免，因为它是默认值。 然而，如果您需要另一个默认值， 然后，由于初始化顺序不受控制， 我将修改如下:

class MyClass extends FooClass 
{
    String a;
    {
        if( a==null ) a="my custom default value";
    }
    ...

There is one more subtle reason to initialize outside the constructor that no one has mentioned before (very specific I must say). If you are using UML tools to generate class diagrams from the code (reverse engineering), most of the tools I believe will note the initialization of Example 1 and will transfer it to a diagram (if you prefer it to show the initial values, like I do). They will not take these initial values from Example 2. Again, this is a very specific reason - if you are working with UML tools, but once I learned that, I am trying to take all my default values outside of constructor unless, as was mentioned before, there is an issue of possible exception throwing or complicated logic.