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Rubyのクラスとインスタンスで特定のメソッドが存在するかどうか調べる方法

前提条件
Ruby: 2.5.1

クラスに対してメソッドの存在を確認する

method_defined?を使う

String.method_defined?(:to_i)
=> true

# to_iメソッドあることがわかるので実行してみる
"1".to_i
=> 1

インスタンスに対してメソッドが存在するか

respond_to?を使う

"string".respond_to?(:to_a)
=> false

# to_aメソッドがないので実行してみるとNoMethodErrorエラーになる
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