只由靜態欄位和靜態方法組成的類，像工具類，這些類不能被例項化，只通過類直接呼叫靜態方法和使用靜態欄位,就將該類的構造方法設為私有的，jdk中java.util.Arrays、java.util.Collections和java.lang.Math都遵循這個原則

public class Arrays {
    // Suppresses default constructor, ensuring non-instantiability.
    private Arrays() {}
}

public class Collections {
    // Suppresses default constructor, ensuring non-instantiability.
    private Collections() {}
}

public final class Math {
    /**
     * Don't let anyone instantiate this class.
     */
    private Math() {}
}