What's the difference between calling Double.valueOf(String s) and new Double(String s)?

Your assumption is right. The second way of getting a Double out of String can be faster because the value may be returned from a cache.

Regarding the second question, you may create a helper null safe method which would return a null instead of throwing NullPointerException.

from apache

public static Double valueOf(String string) throws NumberFormatException {
          return new Double(parseDouble(string));
}

public Double(String string) throws NumberFormatException {
          this(parseDouble(string));
}

from sun[oracle ] jdk

 public Double(String s) throws NumberFormatException {
    // REMIND: this is inefficient
    this(valueOf(s).doubleValue());
    }

public static Double valueOf(double d) {
        return new Double(d);
    }

Depends on the implementation. openJDK 6 b14 uses this implementation of Double(String s):

this(valueOf(s).doubleValue());

So it calls valueOf(String s) internally and must be less efficient compared to calling that method directly.

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